Why use const qualifier in return value?

Question

C++ / C#

Good evening. Everything is clear here: we promise the compiler not to change the object, we return the same that we get (a reference to the object), we forbid changing the structure of the incoming object . Only the purpose of the following prototype is not clear: Namely, the first qualifier. What for in return to specify the qualifier const?
String& String::test(const String&)const;
const String& String::test(const String&)const;

Answer 1

const String& String::test(const String&)const;
What for in return to specify the qualifier const?

In the same place, the link is returned so that the line by reference is not changed.