You put pressure on the center without actually placing a piece there. It gives you the flexibility of attacking elsewhere.
One disadvantage is if white has a pawn on b2 and c3 and castles to the queenside the fianchetto bishop is biting on granite and doing very little. A bishop in the corner can only attack in one direction.
Many people like to use their bishop to put pressure on their opponents knights. A fianchetto bishop is out of the game, so to speak.
What exactly are the advantages and disadvantages when fianchettoing your bishop?