Knives have some potential advantages over firearms as a self-defense weapon. It takes a minimal level of training for a knife to become useful as a self-defense tool. A knife can be used in situations where the use of a firearm isn’t prudent, in a crowd for instance where over-penetration and innocent bystanders may be a concern.
While disarming someone who has a gun is no simple matter; disarming someone with a knife generally involves grabbing a sharp piece of metal and is infinitely more difficult. I don’t know many people, even those who are highly trained, who are willing to attempt a knife disarm.
A knife does not need to be reloaded and short of breaking the blade or catastrophic failure of the locking mechanism a knife does not malfunction.