The zombies mode in Call of Duty: Black Ops Cold War is called Die Maschine, and while it's a "completely new chapter" for the secondary game mode some things will never change, and this is one of them: The Combat Knife absolutely rules. The mode has only been out for a few days, but the players have pretty quickly cottoned on to the fact that you can just stab zombies to death and nobody can stop you. The joke of secondary weapons is an absolute king in the early rounds of zombies mode.
The trick is fancy footwork, keeping moving, and the knife being a one hit kill on normal zombies. Fast players that can dodge hits will get a lot of value out of the knife. You'll get even more if you melee attack while sprinting for the fast lunge attack animation, seen here:
Some players use it as their main weapon up until, or past, round 15. Once you've ventured into the Dark Aether and retrieved the weapon upgrading Pack-A-Punch machine the knife gets, well, it gets really good. It'll carry you into and beyond the late game. An upgraded knife is absolute murder on boss zombies, and a triple-upgraded one can be used by a canny player to hit-and-run whole late game waves.
So, uh, PSA I guess. Now you know. Go stab some zombies.