The vast majority of military color paints are available only as flat paint. Flat paints by nature are rough surfaces, and if you apply decals over flat paint you will suffer the dreaded "silvering" effect of trapping air bubbles beneath the decal. This looks awful, of course! The first gloss coat it merely to prepare the surface for decals. The second gloss coat is to seal the decal which will protect it during the weathering stage, and also has the benefit of making the decal look "painted on". The final flat coat is to return the military color to it's former flat glory and to seal in the weathering effect. There is always the chance that applying the final flat coat will diminish the effect of weathering, so people sometimes lay the weathering on a bit thick to compensate.
Future can indeed be airbrushed with fantastic results. Clean up is a snap since Future is basically nothing more than a clear acrylic paint. I use Windex with ammonia which is nothing more than 67% water, 30% isopropyl alcohol and 3% ammonia. There are a couple of other chemicals in that mix, but in such small amounts that I'm just going to ignore them. If you get the Future on your hands ordinary soap and water will remove it.
If you get a bottle of Tamiya Flat Base X-21 you can add a bit of that to the Future to acheive a very nice flat coat. See Swanny's Complete Future article for more information on how to do that.
Future can be added to acrylic paints to help them airbrush better, or to make translucent paints. It can also be used to adhere decals (see Swanny's article again). It can be used to make mud puddles or dial gauge faces. Dipping clear styrene into Future helps protect it from scratches and has the tendency to make the clear styrene appear "thinner", it certainly glosses it up!
There are also rumors that it can be used to make vinyl floors shine! Though no-one I know would think of wasting it like that.