Answer 1

It's used to access a value which is in another scope. This can be used for various reasons.
If you use a variable the program first searches it in the scope you are and then in other scopes.
Let's say you have a global variable named x, but there is another variable named x in a function. When writing cout << x it outputs the value of the local x. When writing cout << ::x you output the value of the global x.

Another example:
You created a class and just declared the methods. To implement the code of the methods you use the scope resolution operator:

class Foo{
void bar(); //just the declaration
};

//implementation outside of the class
void Foo::bar(){
cout << "blablabla";
}