Why does my road bike come with only one valve adapter? My front tire has the skinny valve (I believe it is a Presta) and so does my rear tire. But my rear tire also has a screw-on piece that goes over the valve which I believe to be an adapter. Do they simply provide one adapter because it is cheaper and you can move it from one tire to the other?
Because you only fill one tire at a time.  The adapter screws off and can be used on either tire if needed.  It can also be easily stored with your patch kit, so it doesn't need to be on the bike.  I only keep one around in case I get stuck with only a schraeder pump at my disposal.