Why Rxb1 ?

Stockfish suggests 26...Rxb1 here. Why would I give up control over the b file and then play Qe2 ?
en.lichess.org/caEMiTfX/black#51

I think without exchanging the rooks some complications might happen with the blacks bishop when he moves to b5, the a3 pawn also might be more free to move forward, while the computer variation white is free to reach his target without worrying about the a3 pawn.

Not sure, but I ran an engine for one hour to compare to stockfish7. It likes Bd8 and Qd3.

Depth=32, 1hr:12min

Analysis by Komodo 10.1 64-bit:

1. -+ (-1.98): 26...Bd8 27.Rxb8 Rxb8 28.Kg1 Qd3 29.Ne1 Qb5 30.Nf3 Qe2 31.Qf1 Qxf1+ 32.Bxf1 Rb3 33.Be2 Bc6 34.Bd1 Rb2 35.Bc1 Rb7 36.Bc2 d3 37.Bxd3 Rb3 38.Be2 Bxe4 39.fxe6 fxe6 40.Nd2 Rg3+ 41.Kf2 Bd5 42.Nf3 c4 43.Be3 Rxh3 44.Rg1 Bxf3 45.Bxf3 Nxe5 46.Be4 Rh2+ 47.Rg2 Nxg4+ 48.Ke2 Rxg2+ 49.Bxg2
2. -+ (-1.71): 26...Qd3 27.Qf1 Qxf1 28.Bxf1 Rxb1 29.Rxb1 Bc6 30.fxe6 fxe6 31.Bc4 Bxe4 32.Bxe6+ Kh8 33.Rf1 Rxa3 34.Bxd7 Bxf3 35.Be6 Bc6 36.Bc4 Rg3 37.Rf2 Be4 38.Bf7 Kh7 39.Bc4 Bb7 40.Be6 Bc6 41.Bf4 Ra3 42.Rf1 Rc3 43.Bd2 Rg3 44.Rf2 g5 45.Ba2 Kg6 46.Bf7+ Kg7
3. -/+ (-1.55): 26...Rxb1 27.Rxb1 Bd8 28.Bg5 Bc7 29.Kg1 Nxe5 30.Qxc5 Nxf3+ 31.Bxf3 Qd3 32.Rb7 Bg3 33.Bg2 Qc3 34.Qxc3 dxc3 35.e5 Rc8 36.Ra7 Bb3 37.fxe6 fxe6 38.Ra8 Rxa8 39.Bxa8 Bxe5 40.Be4 c2 41.Bc1 g5 42.Bxg5 Bb2 43.Bxc2 Bxc2 44.Bxh4 Bxa3 45.Kf2 Kf7 46.Ke2 Be4 47.Bf2 Bd5 48.h4 Bb2 49.h5 Bg7 50.Ke3 Bf6

I think that:
If you play immediately 26.. Qe2 let counterplay to white player, in fact: 27. Bg5! and he takes certainly a pawn recovering the white advantage.
The same problem there isn't if you play before 26 .. Rxb1, in fact:
-27. Qxb1 and the Bishop can't go in g5
-27. Rxb1 Qd3! because if the White play Bg5, the White Queen can't retake with the same counterplay:
(All because the White Rook is currently under attack)
After 27. Rxb1 Qd3! 28. Bg5 Bxg5 29. Qxg5 Nxe5!
I hope I was helpful.

If Rxb1 Bc6 wins pawn maybe. If Qxb1 Rb8 takes Control over line again.

this Rxb1 is strange though, computer evaluation dropped from minus 1,5 to 0 after achja played Qe2. But when Rxb1 then evaluation remains with advantage for black.

We humans get our strength by being more pragmatic in chess, not by being more computed, so I wouldn't pay too much attention to such things, but I'm still curious why, I want to know the reason why evaluation dropped after Qe2.

First, Rxb1 is actually a bad move, it is just that the position is still 'won enough' after. Qd3 is the clearest win in my opinion, then Bd8, then Rxb1 (followed by Bd8).

Second, Qe2 is bad, as it allows Bg5. Qd3 prevents Bg5 because Black then has Nxe5. Bd8 also renders Bg5 useless as Black then has Bc7.

Qd3 is the clearest win because in Rairdens line (see #3) black can improve with 36...g5! after which white is in zugzwang and will soon have to give up the control over c4 or will lose his kingside pawns. After 36...g5 black can operate with mate threats using his Bc6 and Ra3, therefore white can not activate his pieces.