What is the use of holes on kitchen knife?

The holes help to keep food from sticking to the knife when slicing things thinly. Ditto for those knives with the little divots on the sides.

Where are these holes? None of my knives have holes on the blade, unless you mean the serrated edge for a bread knife. But those aren't holes completely surrounded by metal blade.

If you do mean a bread knife, then the serrated edge makes it easier to cut rather than tear the bread or cake or tomatoes, etc.