Promoting to a queen is right so often that the exceptions feel like magic tricks. But they're real: because a knight moves in a way no queen can, promoting to one can deliver a fork or a check that the queen promotion simply doesn't, and once in a while that difference is the whole game.
The other classic underpromotion reason is avoiding stalemate: a new queen sometimes leaves the enemy king with no legal moves and turns a win into a draw, where a modest rook keeps the game alive. The lesson isn't "underpromote often," it's "the promotion square is a real decision, glance before you grab the queen."
The rook on b8 guards e8, so promoting to a queen just loses her. But look at where a knight on e8 would attack.
Promoting to a queen loses to Rxe8. Find the better piece.