Different knives are made for different purposes, and some of those purposes require a knife with a dent. For example, a bread knife often has a serrated edge and a dent near the handle. This allows the bread knife to grip the bread better so that you can saw through it without squishing the bread. Similarly, some vegetable peelers have dents or notches near the blade. These help to grip the vegetable skin better so that you can peel it off in one smooth strip.
So, in short, knives sometimes have dents to help them do their job better!