Why we used const function???

We Used Const function why??

#include<iostream>

using namespace std;

class A

     {

           private:

                    int a=5;

           public:

                int func1() const;

     };

int A::func1() const

     {

           cout<<a;

     }

int main()

     {

          A const a;

          a.func1();

     }

Now here i have a small example of const function................

So, why we need const function 

because a const function can't able to modify the class member variable in it. For example we have 

a meter reading class which reads the value of meter in km in its member variable ""M" now we want to display it so we use use a show () const which is only able to read it and display it.

                                   Let's you want to modify a class member variable in const function so you can use mutable keyword ........