Why we use reference return in assignment operator overloading and not at plus-minus ops?

Question

As I read in books and in the web, in C++ we can overload the \"plus\" or \"minus\" operators with these prototypes (as member functions of a class Money):

Answer 1

Because operator+ and operator- don't act on this object, but return a new object that is the summation (or subtraction) of this object from another.

operator= is different because it's actually assigning something to this object.

operator+= and operator-= would act on this object, and are a closer analog to operator=.