What is the use of making virtual function and why we can not make any function as virtual?

Virtual Functions are the basis for "polymorphism", you should look that up or ask your instructor why polymorphism is important in OOP.

There is nothing from stopping you from declaring *every functions* to be virtual but, depending on the compiler, the object instantiation will be larger for each function so declared.

In brief: because they are two different things.

In longer, consider this, and look how say_hello and say_vhello behave respet to A*.

#include <iostream>

class A
{
public:
    virtual void vhello() 
    { std::cout << "I'm A::vhello" << styd::edl; }
    
    void hello() 
    { std::cout << "I'm A::hello" << styd::edl; }
};

class B: public A
{
public:
    virtual void vhello() 
    { std::cout << "I'm B::vhello" << styd::edl; }
    
    void hello() 
    { std::cout << "I'm B::hello" << styd::edl; }
};

void say_hello(A* pa)
{ pa->hello(); }

void say_vhello(A* pa)
{ pa->vhello(); }

int main()
{
    A a;
    B b;    // remebre B <b>is a</b> A
    
    say_hello(&a);   //call A::hello
    say_hello(&b);   //convert B* to A*, call A::hello
    
    say_vhello(&a);  //seek vtable for vhello, call A::vhello 
    say_vhello(&b);  //convert B* to A*, seek vtable for vhello, call B::vhello
    
    return 0; 
}