Numbers correspond with default key on keyboard for selecting a particular weapon.
The knife is an essential weapon to have on every mission, so be glad the game forces you to keep it with you at all times. Remember to pick up the extra knives, after you've thrown them at somebody. It's always nice to have an extra knife to keep a kill quiet. The knife is creepy, as slashing enemies usually elicits sprays of blood in the direction that you were cutting. The silence of the weapon is why it is effective.