Understanding the usage of Test.isRunningTest()

Test.isRunningTest() :

1.This is not used in test classes but in other classes in order to verify if the call is from a test class.
2.Test.isRunningTest() is used to define whether the code is executed by test class.

Example :

public String Test(){

    // Build the http request

    // Invoke web service call

    String result = '';

    if (!Test.isRunningTest()){

        // Make a real callout since we are not running a test

    } else {

        // A test is running

        result = Response;

    }

    return result;

}