2 Knights against 1 pawn (endgame) Why is it important to leave the opponent's pawn in this situation?

From over 7kk games only 426 ends with KNN against KP, of which 286 games (67.1%) is draw. 





In the two knights endgame, how should we block the pawn to force checkmate? In @SmallChess's answer, GM Samuel seem have made many mistakes... 

http://www.chessgames.com/perl/chessgame?gid=1937249
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.


  http://www.chessgames.com/perl/chessgame?gid=1937249