Why is the ampersand before obj for?

Question

In the following code, why is it OK to add/remove the & before obj?
MyClass operator+(MyClass &obj) {
MyClass res;
res.var= this->var+obj.var;
return res;
}

Answer 1

Thanks! So the difference is after we could change obj.var in the function and whether we could see the changed value after function call.