-L<level> will act as logging.basicConfig(level=level), but on a temporary basis, much like -W sets warning's filters.
If you don't setup logging, or don't want to (cause you're checking some library functions), this will be super handy.
If you do setup logging, it has the same caveats as setting the warning system after you passed -W. (There will be a duplicate handler).
Keep in mind that if you already had loggers with handlers set up, you won't need to use this flag, much like if you modified the default warnings you won't set -W.
It's main use is for debugging purposes in the absent or in addition of logging configuration.