Could you provide an example of how it would be beneficial for an 'expert' to use it? Would it not be more appropriate to say "LOCK_UN shouldn't be used. Period.?"

If several scripts are appending to the end of a file multiple times, they might make use of LOCK_UN to take turns locking the file, seeking to the end, writing to, then unlocking the file. Though in most cases its better just to close, reopen, and lock the file again (e.g., for the purpose of writing log files, especially when log files are cycled).

Another use might be to lock $0 so that no other process can run some part of a script at the same time, but then at some point in the script it might be deemed safe (and beneficial) to allow other processes to run the script and so use LOCK_UN. Again, its a matter of choice whether to use LOCK_UN or just close, reopen, and lock the file. Maybe someone else has better examples...