Overriding vs Virtual

What is the purpose of using the reserved word virtual in front of functions? If I want a child class to override a parent function, I just declare the same function such as void draw(){}.

class Parent { 
public:
    void say() {
        std::cout << "1";
    }
};

class Child : public Parent {
public:
    void say()
    {
        std::cout << "2";
    }
};

int main()
{
    Child* a = new Child();
    a->say();
    return 0;
}

The output is 2.

So again, why would the reserved word virtual be necessary in the header of say() ?