Why C++ copy constructor must use const object?

Question

I understand that when we define a class copy constructor of the class is necessary as Rule of three states. I also notice that the argument of the copy constructor is usual

Answer 1

Copy constructors should not modify the object it is copying from which is why the const is preferred on the other parameter. Both will work, but the const is preferred because it clearly states that the object passed in should not be modified by the function.

const is for the user only. It doesn't exist for the actual executable.