I have a class that has three methods A(), B() and C(). Those methods modify the own instance.

While the methods have to return an instance when the instance is a separate copy (just as Clone()), I got a free choice to return void or the same instance (return this;) when modifying the same instance in the method and not returning any other value.

When deciding for returning the same modified instance, I can do neat method chains like obj.A().B().C();.

Would this be the only reason for doing so?

Is it even okay to modify the own instance and return it, too? Or should it only return a copy and leave the original object as before? Because when returning the same modified instance the user would maybe admit the returned value is a copy, otherwise it would not be returned? If it's okay, what's the best way to clarify such things on the method?