Underpromotion
Promotion to a knight, bishop, or rook is known as an "underpromotion". Although these pieces are less powerful than the queen, there are some situations where it is advantageous to underpromote.
Underpromotion is also sometimes used in casual games as a way of showing off to a significantly inferior opponent. Similar techniques used in these sort of games including sacrificing powerful pieces and putting pieces in a tactically vulnerable situation which the opponent is unlikely to take advantage of. This is considered to be unsporting.
Read more about this topic: Promotion (chess)