You can stab those damn zombies now!
This is very useful specially when zombies haven't seen you. If you stab them in the back, they die instantly. If you stab them in the front, it will take a few hits for you to kill 'em:
To avoiding being heard when you approach a zombie, you have to go stealth. Otherwise they will hear you and try to attack.
Also, if you shoot, all of the zombies nearby will go to the source if the sound.