When do we need to use gcc/g++ to generate Map files? "-g" is enough for debug?

If we want to use gdb to debug a release version executable, we make sure we compile using -g and then use objcopy to trim the debug information. This debug information can be used to locate source file lines when going through source files.

But when/why do we also need to generate Map files, any requirement or ability of gdb relies on Map files?

Thanks.