Why does White fianchetto the light square Bishop in QID 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?
There are 3 reasons why White fianchettos the light square bishop:


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