It store PHP session on it and other application that's require temporary storage of data when the application is running. You do not have worry much for tmp as long as it does not stay over 80% at all time.
Yikes, thats a bad idea. You'd be erasing all the active PHP sessions on visitors to the server and who knows what other applications have temporary files in use. The safest time to clear /tmp is on reboot when no programs are running, and a lot of distros do this already.
If you're gonna clear /tmp at least check if any applications have files in use in it!