It just says if we are compiling a debug mode, output the Logs ALSO to the Debugger output window if the debugger is attached. It does not change the logging behavior. You will still get Log file and Console logging.

Why it is added?
Some log lines are very long and reading them on the console is not easy, and it is not good idea to open the log file while it is open by the engine. The output window gives you the best of both worlds.