Whats the point of abstract methods

Whats the point in making abstract methods if you have to override them anyway in a derived class and you don't even define the body?
it seems like its just a waste of time. I understand what inheretance is and I see why polymorphism is useful. Not all animals are frogs, but all animals eat. I get it. But whats the point of the abstract method if there is no body to it anyway and I have to rewrite everything with override in my new class inheriting the abstract class.