Why was 49...Rb4+ played in this puzzle? I was looking at the answer of today's puzzle in chessgames and see that the reason I wasn't able to solve it because I missed a key move in the solution:

Black played 49...Rb4+
note: this is not the puzzle position, this was taken from the middle of the puzzle solution.
note2: the board seems to be correctly flipped.

8/7p/1pBP2pk/pP6/K6P/6P1/1rp5/4R3 b - - 1 49 


I don't really see the advantage of doing that move, is it something related to winning tempos? If so, how?
Moving the king once like that is a priming move in order for Black to be properly aligned at a crucial point in a few moves.

Without the priming move this happens which allows White to win

8/7p/1pBP2pk/pP6/K6P/6P1/1rp5/4R3 b - - 1 49

1... Rb1 2. d7 Rxe1 3. d8=Q c1=Q 4. Qf8 Kh5 5. Bf3++


So it is desirable to get a check in when promoting in order to win the game:

8/7p/1pBP2pk/pP6/K6P/6P1/1rp5/4R3 b - - 1 49

1... Rb4 2. Ka3 Rb1 3. d7 Rxe1 4. d8=Q c1=Q+ 5. Kb3 Re3+ 6. Qd3 Rxd3+ 7. Ka2 Ra3++


I decided to show a simple mate ending. It is possible for white to prolong the inevitable . This would be done with 3. Be4 . Although it is possible for white to slightly delay, Black still wins with Rb1.