in most cases when a thread is deleted, the answer is in the BBS rules i.e. you should know why it's deleted. some threads, like crappy photoshop/mspaint threads gets deleted because most ppl are sick of them (they're basically spamcatchers anyway in most cases), video-threads might get deleted too without message (obv.).
all cases are unique and up to the individual mod to make a judgement. basically it looks alot better if there aren't as many "red tanks of dooooooom" on the page, it steals attention from the threads that actually are open and have decent discussions in them.
i hope this explains why deleting threads is so fucking awesome :)