I use under painting for a variety of reasons.
Conventionally, it is useful for extremely faint variations in color, implied-texture or reflectivity. In this respect, it is similar to adding a wash over a portion of the dominant color, as it subtly affects the final appearance without adding distinct brush strokes or delineations. I find under painting to be slightly easier to control than washes because the pigments which I use for under painting are less dilute and therefore quicker drying than their wash equivalents.
A less conventional usage is to mark the center point of a zone or to layout a grid upon an area which will ultimately be invisible in the final product, but is useful for determining proportions prior to the addition of the outer obscuring layers. For example, when laying down shadows which are being cast by multiple identical objects and at nearly identical angles to the light source, it is helpful to have an almost invisible grid (like graph paper) present. It helps me be more consistent and to adjust the angles with uniform variation between each of a series of shadow-casting objects.
My final use for under painting is for low risk experimentation. Laying out a rust pattern or a dirty spot on an otherwise pristine rendered subject is daunting. You've finally got the thing looking exactly like you want and now you just need to scuff it up a bit to make it look more real. But a little too much or even a little in the wrong spot will ruin your hard won rendering. These are the moments that make brush hands quiver.
Which is why I design my rust and scuffing before hand, to get it right before I add all the pretty paint and shine. Then, if I have to go back and strengthen the abused area after it has become too pretty, I have the still barely visible tone of the under painting ( or at least my memory of it ) to guide my hand.