When do I use '#include <string>' at the start of a C++ program?

I am confused about the use of #include <string> at the start of a program. For example, in the code below, I don't use #include <string> but the function will still print out the string "Johnny's favorite number is" when it is run.

#include <iostream>
using namespace std;

void printVariable(int number){
    cout << "Johnny's favorite number is" << number << endl
}

However, in this code below, it does contain #include <string>.

#include <iostream>
#include <string>
using namespace std;

class Var{
    public:
        void setName(string x){
            name = x;
        }

        string getName(){
           return name;
        }
   private:
       string name;
};

int main(){
    Var Classy;
    Classy.setName("Johnny Bravo");
    cout << Classy.getName() << endl;
    return 0;
}

Do I only use #include <string> if a variable represents a string?

Do I only use #include <string> if a variable represents a string?

Yes.

Use #include <string> when you use a variable that has type std::string.

The code "text here", contrary to intuition, is not a std::string; it is a string literal, and a C-style string, and a const char[10] convertible to const char*. Welcome to C++ with its legacy oddities.