This prevents unintentionally calling the function of the base class.

class Base
{
public: // Methods
    virtual void f() const;
};

class Derived : public Base
{
public: // Methods
    virtual void f(); //This is not an override, not allowed !!!
};

The reason is when a client attempts a polymorphic call to f() through a const pointer or const reference to Base, it will call Base::f() and not Derived::f(), which is syntactically correct but probably surprising. The same care should be used when overriding a function with a parameter that is a pointer or reference to const.