Some excerpts (rather one half, emphases by me) from the introduction to the book by Thomas Fairland, in which the usefulness of the images is made clear:
The system endeavoured to be illustrated in the accompanying Plates, by a progressive course of figures, is that essential method of decidedly determining the relative bearings of all the most conspicuous points in a geometrical manner, before proceeding with details, as is indicated in the early examples by dotted lines; nothing is arranged or recommended in the copying of the present subjects, but upon principles which can be equally and truly applied to the delineation of the real objects, and therefore the learner may rely with certainty on the foundation these will become for subsequent practice.
Having mastered [the single features of the face], the full face may then occupy his attention; and in this a few mechanical rules will assist him: as, for instance, in attempting the entire face he may first describe an oval, somewhat narrower towards the bottom than at the top, when he is at liberty to draw, or suppose to be drawn, a perpendicular line down the middle, which serves as a guide for the situation of the nose and mouth ; this line is to be crossed by others, on which are placed the nostrils, mouth, and chin. In every variety of position the existence or supposition of these lines is of great service to the learner, as they enable him to place the features much more accurately than could be done without an adherence to them; always remembering that in every case the lines on -which the eyes, nostrils, mouth, and chin are placed, must perfectly correspond with each other, and be always at right angles -with the perpendicular line supposed to be drawn through their centre: the continuance of this method will more readily produce accuracy, and undoubtedly render the practice more easy and agreeable to the beginner.
In other words, the reference lines are there to get a feeling for the relative proportions, directions, and volumes, and can be used both as physical guides in your drawings or as purely visual aids.