There are many reasons why posts can be deleted, some are obvious and others are touch and go. The touch and go posts will usually be left until some one reports them and there are those posts that do not get spotted, for what ever reason..see rules
Rules
One of the obvious one’s to spot are the ones that hi-jack threads.
What happens is that someone will start a thread with whatever topic and for a while things go along fine without any problems. Sometimes, as mentioned earlier some threads do wander a little and nobody really minds.
What does spoil it i.e. the thread, is when one or two maybe more members take over the thread by chit-chatting amongst them selves on a none related subject, such as “are you going out tonight” “lol yes. Are you?” etc etc. I am sure that this sort of chit-chat is very important to the chit-chatters involved but to the rest of the members trying to follow the original topic it means that they have to scroll through loads of rubbish to find the next relevant post to the original topic.
So if these chit-chat posts are not removed then it could possibly kill that thread because others might not want to be bothered having to scroll though all the chit-chat posts.
I’ve seen in a report that there were nearly 40 chit-chat posts deleted in one thread alone, now imagine that.
If members want to have a chat they can either use the chat room or PM each other thus leave the threads clear, this way their posts will not be deleted.
I hope this helps.