Endgame where two knights against a lone king is a draw, you can't force a checkmate. However, with a pawn the side with the two knights can setup a mating net without running into stalemate. That's why it's always important to leave a pawn.
Just a few days ago, Karjakin demonstrated the technique against a very strong US grandmaster. Please note in the final position, it's a stalemate without the black pawn. Please go through the game, note how Karjakin never attempted to win the final pawn.
White is threatening Nh6, Nf7#. Again, this setup is impossible without the black pawn.