What really is the purpose of “base” keyword in c#?

The base keyword is used to refer to the base class when chaining constructors or when you want to access a member (method, property, anything) in the base class that has been overridden or hidden in the current class. For example,

class A {
    protected virtual void Foo() {
        Console.WriteLine("I'm A");
    }
}

class B : A {
    protected override void Foo() {
        Console.WriteLine("I'm B");
    }

    public void Bar() {
        Foo();
        base.Foo();
    }
}

With these definitions,

new B().Bar();

would output

I'm B
I'm A