Pawn promotion question from a newbie

I'm trying to figure out if there is really any reason why someone would promote a pawn to a piece that isn't a queen.  I suppose if by promoting it to a knight then and there would cause a checkmate I could understand, but I'm just not sure the point behind promoting it to a rook or a bishop since the queen is in essence both a rook and a bishop.

For starters, it would be easy to compose a position were if you promote to a queen, you have a stalemate, but promoting to a rook is a win.

In this composed problem, one of my favorites, White MUST promote to knight and THEN a bishop and THEN a rook.  That's the only way to win.

Yes, that's three under-promotions, all in the same problem!

http://www.edcollins.com/chess/under-promote.htm

White to play and mate in 7.  (White must promote to a rook.  That's the only move that wins.)