Black squared bishop in Accelerated Dragon

I was studying the very good video lessons about Accelerated dragon.The GM said, a lot of thimes, in Marzocky positions (e and c4 for white) that black should exchage the "dragon bishop" as fast as possible.But he did not say why and, putting in an engine, the position, the engine "likes" the exchange...

this thread is useless without a diagram.

generally though, it's well known that in the Maroczy white's dark squares can become really weak, so it's white who should hold on to his bishop.

Are you talking about allowing it to be exchanged or are you somehow forcing it off the board?

1.e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 g6 5. c4 Bg7 6. Be3 Nf6 7. Nc3 d6 8. Be2 O-O 9. O-O Bd7 10. Rc1 Nxd4 11. Bxd4 Bc6 12. f3 a5 13. b3 Nc7 14. Bxg7 Kxg7 is equal, but the exchange was not forced and 14. Be3 Nc5 15. Qd2 Qb6 is the mainline. 

Sorry.I found the game and will put the diagram.The Db shows that really, almost all the GM's that played this with black pieces, made this trying to exchange the dark color bishop and so the video (GM Milikset) teachs.I'd like to understand why...

Mainly it is because White's dark squares are weak. His pawns are semi-stuck on light squares leaving the g1-a7 diagonal incontestable, at least without exchanging queens. The second point is to avoid the good knight bad bishop position. If Black could, he almost never can, he would exchange all of the pieces off except for his knight and White's light square bishop.

The computer prefers the exhange? Does it follow up with Kh1 or does it allow Qb6?

This is what I believe - the dsb exchange generally favours Black since White's dark squares are so weak. Care needs to be taken, though, that the exchange isn't merely a prelude to White breaking the position open - then Black might prefer to have his dsb for defence.