Why does White fianchetto the light square Bishop in QID

Question

The line for Queen's Indian Defence goes like this -

[FEN ""]    
1. d4 Nf6 2. c4 e6 3. Nf3 b6 

Now, the most popular choice for white in this position is g3 preparing to fianchetto its light squared Bishop. After this, if Black wants to develop its light squared Bishop, the two choices are either Ba6(modern main line) or Bb7(old min line). Suppose Bb7 is played by Black. Now white can also fianchetto its Bishop and the play can go on. But here is my question - Given that Black plays b6, why does white go on with g3 ? Given that white's main light squares defender around the king (after white castles king side) is its Bishop on g2 which at some point in the game can be exchanged with Black's bishop on b7, isn't this line a bit more riskier?

However, this line has been played at top level a very large number of times. Like yesterday in Tata steel 2017 masters, Aronian played this opening and won a wonderful game. So what is the justification behind white's fianchettoing its light squared Bishop given Black has already done that?

Answer 1

There are 3 reasons why White fianchettos the light square bishop:

1. White's plan is to play e4 to gain space. The bishop on g2 supports this
2. Building from point 1), white could play e3, Bd3 then e4, but that is an extra tempo spent to play e4
3. White has latent pressure on the queen-side, and may be able to play discovery or pinning tactics with a bishop on g2

Answer 2

Given that white's main light squares defender around the king (after white castles king side) is its Bishop on g2 which at some point in the game can be exchanged with Black's bishop on b7, isn't this line a bit more riskier?

If they get exchanged black will also lose his light squared bishop which could have taken advantage of the weakened light squares around the white king. For white it would be much worse to exchange the bishop on g2 for a black knight which could make that bishop on b7 very strong.

In any case there is nothing wrong having such opposing bishops. Many other factors play a role here as well and the white bishop on g2 does put some pressure on the center and is not threatened to be immediately exchanged. You really need to look at the whole position not at one element only.

Otherwise you could argue that any opening where you fianchetto the bishop and castle to the same side (King's Indian, Grünfeld, ....) is bad as the opponent could always fianchetto himself to oppose your bishop.

Answer 3

The reason is the opposition of influence in the center and the common break of e4 by White. Black's q-side fianchetto by b6/Bb7 met by g3/Bg2 is common and for good reason. White wants to oppose the Bb7 with his bishop on the same diagonal, and in turn try to get in the break e4. With c4/d4 played, White will likely go for a queenside assault, perhaps down the c-file. With the b6/Bb7 being played, the fianchetto by white makes the white Kingside more secure and contests the long diagonal.