An adjustable gas block lets you tune the gas system to the loads you're using. With hotter loads you'd close up the aperture a bit to keep your brass from being tossed into the next county, and with softer loads you'd open it up so it will cycle more reliably. Or you could close it completely to turn it into a single shot/straight pull bolt action.
It's a fairly popular mod for Mini-14s, as they tend to have problems with chucking brass all the way across the range, but I haven't heard of them being used with AR-15s all that much.
It may be worth buying if you plan on mounting a suppressor any time in the future, as closing the gas system helps keep the noise down or you can open it up and cycle subsonic loads better, but other than that it's not really all that necessary, unless you absolutely hate chasing brass down.
Last edited by Regolith; September 29, 2012 at 08:38 PM.