We move posts around if they are off-topic for the forum where they're originally posted. If we left all of the off-topic posts we would have a bunch of forums with a mishmash of thread subjects discussed by the same group of forum regulars, and new people would have a hard time figuring out where to go. We leave redirects so people can see where their threads were moved; if others want to comment on the thread they can always follow the redirect to do so.

Some people tend to only visit one forum and want all of their posts to stay in that forum whether they're relevant or not. This can lead to feelings of possessiveness about "their" forum and make it difficult for new people to feel welcome, and we get complaints about it.

Also, some topics can get very heated (politics, helmets, A&S stuff) and they are easier for the staff to monitor if they are in their own forum rather than scattered about in different areas.