The only reason for which someone wants to delete their post history is to cover their stupid mistakes, wants privacy etc though that's not possible due to some of the archival services.
I think you are right. A user may have made some multiple mistakes several times which may be caused by the user being new to the forum, typographical error, wrong information, wrong date etc and once the user is able to get the knowledge of the past mistakes he or she has made in the past that person can decide to delete it but deleting a post doesn't really mean the information is out completely as you earlier said the post can be found in the archival services but looking at the information contained in the post of the op, I think there is more to some users just deleting their posts than what we are thinking about.
You guys are definitely right, I mean like i stated in the op, for us who are not in the culprits shoe, we can only imagine a lot of different reasons why a user might decide to delete some of their posts, but in all of those imaginations, i am very sorry to say that a user choosing to delete some of their posts simply because of some sort of mistakes they made when they were newbies is the lamest of all, like i said before, I am sorry for seeing it this way but its just my opinion.
Yeah, Not every user looks at the forum's achieves sure thing, I myself probably have never looked there, so if you delete some of your posts for whatever reason, not everyone will call it to mind to look at the forum achieves to find out, this simply means that many will never find out.
And again coming back to mistakes made when the user was a newbie, I myself was for a very long time a bounty hunter before I even understood that there was more to this forum than just bounty hunting, I probably have made some mistakes as a newbie and bounty hunter, if no body ever asked me about those mistakes, even the managers of the different signature campaigns I've been a participant in never asked me before accepting me, or rejecting me with a note that my past mistakes is the reason why i was rejected, if none of this ever happened, what credibility or reputation does deleting this past posts add to me now as a user of this forum?