Why Bb6 instead of Ba7?

It makes no sense to me and most masters choose Bb6 instead of Bb7.

I am following Mauricio Flores Rios' Chess Structures book and in the position after

PGN: 1. e4 e6 2. d4 d5 3. Nd2 c5 4. Ngf3 a6 5. exd5 exd5 6. dxc5 Bxc5 7. Nb3
FEN: rnbqk1nr/1p3ppp/p7/2bp4/8/1N3N2/PPP2PPP/R1BQKB1R b KQkq - 1 7

54% of masters (from the lichess db) choose Bb6. Only 29% choose Ba7.

To me it seems to only:

1. block the b-pawn
2. keep the dark-squared bishop more exposed
3. prevent the c8 Bishop from ever fianchettoing on b7

with absolutely no added benefit. Can someone please explain?

I don't play this variation but to me the bishop has more scope on b6 and keeps an eye on Nb3. With the IQP I'd be looking to develop Bc8 along the c8-h3 diagonal rather than a fianchetto.

I will put my bishop at b4 and trade it of early in the opening.

Probably a7 is too inflexible in an open position like the IQP. From b6 the bishop can go to c7 for example, where it fights for central squares and could be used for an attack when White castles.

As @lizani noted Black would not want to fianchetto his light-squared bishop—way too slow in an IQP!

It’s to play Bc7 and form a battery with Qd6. Also a fianchettoed bishop on b7 just stares at the b7 pawn.

Bb6 restricts the Nb3.

Ba7 does not to the same extent.

That's about it.