See the pitcher plant drawing here, where they have actually drawn the wireframe first (see image below).
What will I gain by drawing the wireframe first? Why not draw the outer lines directly?
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The reason for drawing wire frames, or any construction method, is to properly develop the forms of the object. By laying out the underlying structure, you can get a better sense of volume, proportions and arrangement of the 3D object you're trying to map into a 2D art form. This lets you get the foundation ready before laying down time-consuming details that may be hard to fix later.
If you focus first on the outline, you might recognize the plant as a plant, but you'll lose out on the understanding of the curved blades that make the leaves, or the cylindrical shape of the pot.
Generally speaking, if you start from the edges, the following mistakes become common:
These problems can leave the drawing looking dimensionless and misshapen.
Additionally, as you practice construction of real objects, you develop your natural understanding of how objects work. This allows you to become increasingly capable of constructing objects from imagination, or filling in blanks of objects you can't fully see. This is a skill that only comes about with practice.
Now, there are methods of practice and application where you focus on drawing the outline. One exercise is drawing the negative space around an object, effectively leaving you with a silhouette or outline. In such a case, you focus strongly on the relationship of angles and distances between empty spaces. Here, you're also training your eye to interpret 3D objects as 2D forms.
This is also an essential skill. So to answer your question about whether it's always necessary to draw the wireframe first, I would say no. Specifically, learning to draw negative space is highly valuable, and can be combined with your understanding of construction to make believable objects.
It is not necessary.
You can draw anything without any manual compositional planning whatsoever! However...
Freehand Drawing is HARD.
If you can sit back comfortably, keeping your whole panel in view during the drawing process, have an excellent eye and a highly practiced hand, have a lot of talent and experience, and just a little luck, you can avoid mistakes. (Or at least cover up the little ones) This is not easy, requiring decades of practice to be done consistently. Thus, it is far more practical to first manually plan one's composition using guidelines, contour lines, line of action, perspective, and other forms of composition construction lines. It bypasses the demand of practicing freehand for 15+ years, allowing the artist to become technically competent within weeks of dedicated practice, ensuring consistent results. There are also really only two reasons to practice freehand drawing: performance art, and ink drawing.
In a final product, nobody can tell whether you drew it freehand or used guidelines to construct your composition. (Except for crazy art lovers, but many of those people actively want to see traces of the construction in the finished work) As such, the only reason you would want to do freehand a drawing with dry media, is if someone is watching you draw it and you want to be committing every mark to a final product. Freehanding can be very impressive to watch, and can earn good tips. As such, it's pretty much only used by street artists, like caricaturists.
Meanwhile, when working with ink, (and some other permanent media, like metalpoint) it can be detrimental to the finished product to use guidelines. Ink does not stick to graphite well. Colored inks will mix with charcoal. Anything between an ink and the paper can cause it to fail to bind correctly, and even make localized changes in bleed. Careless guidelining can damage the paper's tooth, affecting the appearance of the ink mark. Erasers ruin the altered surface. Some inks include a lacquer in their binder, which traps your guidelines and exposes them forever. If you're working in brushed ink, particularly in an eastern calligraphy tradition, the idea of guidelines goes against the very philosophy of the art form. There are ways around these issues, but they require intimate understanding of the physical properties of your media and paper, and result in serious restrictions on material selection. In these media, technical mastery depends on one learning to freehand with a permanent mark.
There are basically two ways to draw an object:
Drawing what you see.
Drawing from reality as you see it can result in strikingly "photorealistic" renderings. But it has two fundamental disadvantages: You cannot draw what you cannot see. For example, a dragon cannot be drawn from reality, but any other object will prove equally undrawable, if you don't have it in your vicinity.
And sometimes reality is not as "attractive" or "ideal" as you might want your finished artwork to look. In fact, sometimes reality looks plain wrong and viewers of your artworks will tell you that you made a mistake, when what you drew looks exactly like it does on a photograph you took at the same time and from the same angle. For example, many poses in nude figure drawing look as if you cannot draw when you draw them exactly as they appear in front of you, because we are so used to the idealization in highly selected and professionally posed and lit nude photography and aren't very familiar with real naked persons at all.
Drawing what you know.
Drawing from the imagination can result in beautifully elegant renderings. But is has two fundamental disadvantages: You cannot draw what you do not understand. For example, a complex tangle of branches might prove too difficult (or time consuming) to construct convincingly. And purely constructed renderings will often appear stiff and lifeless. For example, a constructed figure may appear unbalanced or as if it does not interact with its environment (e.g. the flesh does not adapt to the chair).
From my experience, both approaches eventually lead to the same skill: that of being able to draw from your imagination by referring to references from reality.
The "constructing" approach will provide you with knowledge quickly (i.e. you learn proportions and other constructing principles that you can immediately apply) – which makes it appear as the faster and better (and sometimes the only correct) approach to some students –, but it has to be constantly checked against reality so that your drawings don't look like stacked cubes and spheres but have the dynamic and force of real persons, animals, and objects.
The "observing" approach will provide you with motor skills quickly (i.e. you learn how to move your hand to create a line that gives the impression of something real), but it has to be abstracted so that you may escape the limitations of perceivable reality.
In essence, you always do both things. When you draw from reality, you either explicitly/consciously or implicity/unconsciously begin to form an understanding of the structural principles of the objects you draw. If you draw from life long enough, you will eventually know what people etc. look like and be able to draw them from your mind without every having learned proportion or wireframes. You can speed up this learning process by not only drawing what you see, but by consciously attempting to draw the abstractions. There are many kinds of exercises for this, and drawing wireframes is one of them. (Another is redrawing what you have drawn from life from your mind, which will force your mind to create its own abstractions and its own structural principles. This is the approach I prefer to learning someone else's abstractions, which, I have found, usually do not fit my personality and thinking.)
When you construct from structural principles, which you correct against real life references, you will eventually incorporate reality into your abstract knowledge. The danger in learning to construct first is that you learn only the construction and check your art against the constructing principle (e.g. schemata of proportions) or the work of other artists (e.g. the comic fan copies comics).
To finally answer your question, it does not matter whether you draw the wireframe (that is, the constructing principle) first or the outline (that is, what you see). You can even do one or the other exclusively for a long period of time. The important part is that you keep drawing (this is the most important part, actually, and how you draw matters to a lesser degree), and that you eventually begin to see the limitations of whichever approach you chose and begin to supplement it by doing the other thing.
That is, if you construct, construct from life. And if you draw from life, find the general abstract construction principles underlying reality.