This was one of my first questions after I understood the assembly of the AR-15.  I’ve never owned a semi-automatic rifle before… but I’m guessing this gas acts upon something in the upper receiver assisting with the loading of a new round in the chamber. I did some digging and found this REALLY in depth article on the gas system.  Link to the article below.

Source: How the AR-15 Direct Impingement Gas System Works

In short, the bolt carrier group needs help cycling.  The recoil from the .233 round isn’t strong enough to move the bolt far back enough to eject the old round and scoop up the new one.  By taking pressure from the barrel, and delivering it to the BCG via a Gas Block & Gas Tube, the thrust is enough to force the whole assembly backwards far enough.  Whenever I meet someone whose AR isn’t cycling correctly, the first place to check is the gas system.

Some people feel this type of semi-auto system is flawed… however, it doesn’t appear that there is much proof in the pudding with that argument.

http://www.defensereview.com/the-big-m4-myth-fouling-caused-by-the-direct-impingement-gas-system-makes-the-m4-unreliable/