As an example, say there's a case of flaming, trolling, or harassment where someone is being maliciously targeted. That violates forum policy. If the aggressor in this case deletes all his related posts, it can be cumbersome to retrieve that data and make a fair reasonable judgement based on it. If the post is entirely purged, it may be impossible. Then you slide down the slippery slope of "he said, she said" arguments. The idea is that mods should have good evidence to look at when resolving conflicts. If they don't have good data, good decision-making gets a lot harder.



TL;DR - At the very least the posts should be cached in some way, so they can be looked at later for conflict resolution purposes.