Echoing those above, I use drybrushing for three purposes. In general order of application, they are:
1) What I call "toning." This is like what Don said about pastels and washes. It's a subtle, wide-area effect to "tone" a base color with lighter and darker versions of the base color. It breaks up the montone of a single color on a large area, and helps to mimic the subtle variations of color the eye sees on full-scale objects, due to atmosphere, lighting effects, and so on. It's essentially a hand-brushed version of what a lot of folks do with an airbrush. (I just find drybrushing easier to deal with and control.)
2) Weathering. I start this with a less-subtle version of the above, with significantly lighter or darker versions of the base color (to simulate wear, sun fading, and general grime), then attack specific areas with mud and grime. I also use it after washes to sort of even things out.
3) To make detail "pop" out, essentially adding highlights. This can replicate paint wear on corners and grab handles of metallic surfaces (done with, say, steel), or just highlights on raised surfaces like cockpit gauges, ejection seats, etc. This is generally a very lightened base color, or white, best done by dipping the brush then rubbing away MOST of the paint on a paper towel, then just "flicking" the edge of whatever you're working on. (For best results in this, keep in mind the general direction light is coming from. White highlights on something that should be in shadow can look strange.)
Two more observations, tools-and-technique-wise (based solely on my own experience).
I almost always use enamels or oils for drybrushing, though (these days) most of the rest of my painting is done with acrylics. Acrylics "set" too quickly, especially in the tiny amounts used for drybrushing, and I've never been able to make them work well for drybrushing purposes. Enamels offer much more conrol and subtlety.
After trying bunches of brushes over the years, of widely different quality and materials, I always find myself going back to the cheapo Testors nylon brushes for drybrushing. They're sturdy, don't shed bristles, and since they're inexpensive you don't feel bad about squashing them, or occasionally sacrificing one where required. The nylon bristles seem to be the right mix of stiffness and flexibility, and I find through repeated use that I can "shape" the nylon bristles in useful and predictable ways. I've got a couple of each (wide, flat and pointed/wedge) that I use for nothing but drybrushing, and even with fairly regular use they tend to last a year or two.
The key to drybrushing is PRACTICE, a lot of it. The first time you highlight an instrument panel and it "pops." it's like magic. But there will be a lot of frustration along the way, as well. Just keep at it, keep trying different uses and applications, and it gets to be very satisfying.
Good luck.
Greg