Why Do You Use A Scope Resolution Operator When Defining A Class' Method?

My question about the Scope Resolution Operator (::) is why do we use it in a CPP file to define the methods of a class? I'm more so asking about the SRO itself, rather than the relationship between CPP and Header files.

When you define a class:

struct foo {
    void bar() {}
};

Then the full name of bar is ::foo::bar. The leading :: to refer to the global namespace can often be omitted. There is no bar in the global namespace, hence bar alone (or ::bar) does not name an entity and when you define the method out of line you need to tell what bar you mean:

 struct foo { 
       void bar();
 };
 struct baz {
       void bar();
 };

 void bar() {}    // this defines a completely unrelated free function called bar

 void foo::bar() {} // defines foo::bar
 void baz::bar() {} // defines baz::bar

You need the scope resolution operator to state which method you want to define.

For more details I refer you to https://en.cppreference.com/w/cpp/language/lookup