Why do we need Knights?

None of us would have an answer to this - chess was invented centuries before any of us.

The knight is the only piece that can jump. It can jump over pawns, and therefore it's the only piece (not including pawns) that can make a move in the first move of a game, for either side. Still, chess doesn't need knights.
But I like knights and all the normal chess pieces so much, that when I play variant games (like this), I always play on a 10x8 board (or bigger) so that new pieces can be added, without getting rid of the knights.