Tal v. Simagin - Why Hang the Bishop?

I was watching a commentary on youtube on a Soviet championship game where Tal sacrificed everything and beat Simagin with a passed pawn. 

Black to move goes Rae8. Now up until this point, white was chasing the black king all over the board but black halted the mating attack. After Rae8, white plays Rxg7 and eventually wins once the queen mops up the bishop and pawns and white is able to queen a pawn. 

So am I missing something here? Rae8 isn't stopping mate or threatening mate, why would black not move the bishop first in this position? I can see black not anticipating the sacrifice but he was still going to lose the bishop. 

I just feel like something must have gone over my head here, but I don't see grandmasters hang pieces for no reason. 

Let me show you why.  Double check what i gave.  I may be wrong.

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My viariation is wrong.  It is still a checkmate or white lose the queen and game.  I used the wrong rook in my last move.

Rae8 is the best of a bad situation.

1...Be7+ 2. g5 Bg5+ 3. Kh5 Rae8 4.Qg6 Rh3+ 5.Kg4 Rh4+ 6.Kf3 Re3+ 7.Kf2 Rf4+ 8.Rf4+ Bf4 9.Qc6

1...Bd6 2.g5 hg 3.Kh5 Rae8 4.Qd7 Bf8 5.Rf3 R3e7 6.Qf5

1...Bb6 2.Qg6 Bd8+ 3.g5 Bg5+ 4.Kg4 Bf6 5.Rf6