Sometimes you can't capture a piece to remove a defender, but you can distract that piece. Deflection is the tactic of removing a defender by making it move to another square. Often this involves a sacrifice that will be worth it for checkmate or greater material gains.
Winning the rook can only be done by removing the knight protecting it.
Since it was a check with the knight, black was forced to take with the pawn, damage his structure and leave the bishop hanging.