[numer_05]

Doesn't g++ already tell you if you have errors?

I believe you mean C++. But as for the tool, C++ does tell you if you have an error building the program, but it doesn't tell you if it does what you want or not. Say you want to print A and you've written everything correctly and it builds, but you accidentally have a typo and it prints B.

You then go and check the code to make sure it is correct. Make a few changes and compile again. Basic troubleshooting 101.