If your opponent had played Rf6 it would have forced your Queen to retreat back to the square it just came from, losing time (if you try to move the Queen forward, your opponent can force a Queen trade). The rook sacrifice suggested by the engine removes a piece defending the King and draws the King toward the center of the board where you can set up a mate net with your other rook.
Help with analysis please

Move 15 is a blunder not because it was a bad move but cause there was a really nice move which was Rxe6, king couldn't take back cause then it would be a forced mate in a couple moves. If he doesn't take that's just free material.

When you said in the opening post "I don't see it", what did you mean by that? I'm just curious.
I just meant that at the time I didn't see how it was not the optimal move(in the game it turned out to be a good move), I hadn't yet comprehended how the analysis explains things. I get it now though.
https://www.chess.com/analysis/game/live/37580382227
Can anyone tell me why it says move 15 is a blunder? I don't see it, or maybe I'm just misunderstanding something here....
Thanks