That's a good question. Stats seem to favour g3 for white. Never the less, as white I've resisted playing g3 because I've always felt I didn't understand "why" I was playing it. As black I was always happy to see g3 because it presents a target for the f4 push at some stage.
I'd bet though it as something to do with pressure on the d5 square.
In my search for active, exciting, and/or unorthodox responses to d4, the Dutch Defense seems to be the opening that makes for the most exciting game for me.
My question is: why do white's responses to the Dutch so often include g3 and a kingside fianchetto? This practice seems much more prevelant in the Dutch than in other defenses I've tried (QGD Cambridge Springs, Lasker Defense, Slav, Benko). Not that I'm complaining; I actually enjoy playing against the fianchetto. But why is it so popular?