As with any other function, a member function (virtual or otherwise) can be overloaded. A derived class can override zero or more instances of the overloaded functions it inherits. If a derived class wants to make all the overloaded versions available through its type, then it must override all of them or none of them.
Sometimes a class needs to override some, but not all, of the functions in an overloaded set. It would be tedious in such cases to have to override every base-class version in order to override the ones that the class needs to specialize.