You don't see how lying about when something was posted can lead to problems? Just making sure I'm answering the right question.
The spectrum begins at me being slightly annoyed because I can't tell posts that were posted just now from posts that were posted hours ago.
It probably ends at people arguing in court over such things as prior art in patent cases, or someone trying to submit an online comment as evidence. Imagine Facebook mucking with timestamps... Often when is pretty important.
There's a lot of things in between there which I'll leave to your imagination.
In any case, changing meta information silently can be as bad as silently changing the content of posts themselves and will hurt your credibility as a platform. The reddit CEO did it as a joke once, and that already caused a massive shitstorm[1].
This is a process HN has devised to give selected submissions another chance at the front page. See:
https://news.ycombinator.com/item?id=11662380