Due in part to the over regulation of firearms, Knives are becoming more and more popular for self defense. Knives have some specific strengths that make them a very useful self defense tool.

Knives have less restrictions than firearms. The legal length for a knife is different in many jurisdictions, and there are places where knives are prohibited but it is nothing like the rules surrounding firearms.

Knives can be used effectively with a small amount of training and practice. I am not talking about 'dueling' or knife on knife fighting, but rather employing and using a knife in a self defense situation doesn't require too much training to be effective.

Knife techniques are specific to the type of knife. While there are a number of knife martial arts, many of the techniques only apply to a certain type of knife. If you look at your kitchen knives the cleaver is just the thing for cutting through bone, while the filet knife is just the thing for cleaning fish. The same holds true for defensive knives. I would not expect to be able to use a machete the same way I would use a three inch pocket knife.

Knives are effective in situations where other tools are not. Knives are very effective in grappling situations. Many police officers carry knives to augment their handgun retention. In the middle of a wrestling match it is very difficult to find a safe backstop for your firearm, or even find enough space to get it deployed.

Knives are lethal force. So the situation in which a knife should be used must satisfy all the criteria for using deadly force.