take /var/log as example. if someone floods your server with traffic and you log every tiny bit (or even worse you have the debug level set to on) the logs will quickly take lots of space. if the space is full no further write access is possible
if it's a seperate partition you can't write more logs... if it's on the same partition you won't be able to log in by ssh as example because your drive is full and you can't spawn the sshd agent ;)
This one is worth doing. I've been asked to fix a Trixbox that was blocking calls because they were logging every phone call and it filled up the harddrive. Easy solution to a possibly annoying problem.
take /var/log as example. if someone floods your server with traffic and you log every tiny bit (or even worse you have the debug level set to on) the logs will quickly take lots of space. if the space is full no further write access is possible
if it's a seperate partition you can't write more logs... if it's on the same partition you won't be able to log in by ssh as example because your drive is full and you can't spawn the sshd agent ;)