Using const is useful for several reasons: It enlists the compilers help in ensuring values that shouldn't be changed aren't changed (the compiler will throw an error if you try, like in the above example). It tells the programmer that the function won't change the value of the argument. This can help with debugging.

Why we are pass const reference?

We pass by const reference to avoid making a copy of the object. When you pass a const reference, you pass a pointer (references are pointers with extra sugar to make them taste less bitter). And assuming the object is trivial to copy, of course.

What is one benefit of declaring the parameter as a const reference?

References used as function parameters can also be const. This allows us to access the argument without making a copy of it, while guaranteeing that the function will not change the value being referenced.