Why is the opening common even among GMs?

I've been taking chess a bit more recently recently, and I've seen this opening from time to time. I think I remember Magnus doing it once before too.

Why would black move it's bishop to g4? I've done this many times before (more on impulse really, just to see what would happen), and white almost always makes the same move - h3. Then black usually just retreats the bishop. In retrospect, this is quite obviously what would happen, so isn't this a wasted move for black? Or is there some hidden advantage that black gains by white playing h3?

• 2...Bg4 is called Anti-Torre Attack in the sense of the reversed version of Torre attack.
• 3. h3 is not the best move; 3. Ne5 or 3. c4 are better.
• 3...Bd7 is a bad move; 3...Bxf3 is better.

i think instead of playing bd7 black should play bH5,then sometimes (specially in blitz game) white attacks the bishop again by playing g4 .black then again moves his bishop to g6.

in this way whites kings side castling is made weak and black can hope that the player with white pieces will castle queen side,

(thnx for replying in my forum)