I've seen many user deleted posts with various reasons in the past. Quite often discussions can get heated and someone might say something they wish they had not. They'll delete their post and give a reason like "I shouldn't have said that, apologies". In which case it can show members self moderating, which can be a positive thing for all to see. Out of 100s of posts a day I doubt many get deleted by members at all. As matt said it can also help moderators pick up on scammy behaviour. Maybe someone makes a deal in the classifieds then deletes it. Then there is a breadcrumb to follow later if needed.
Cynic in me would say I doubt anyone is likely to leave an apology for their bad behavior in a crumb that is for everyone to see for eternity. In any case, it's your prerogative ultimately, so this is my last post on the subject.
If some fellow got posters-remorse I consider it enough if he deletes it. I do not expect a public apology. If he wants to, the deletion-note does not leave much space. He could make any apologizing post instead. A chance that always existed at this forum.
You don't have to leave a reason. You can leave a reason, if it makes sense to do so. Examples might be where a post has unintended future effects, and you want people who might remember it to understand that you have had further thoughts on the matter.
Us moderators usually make a point of leaving a reason.
This is the box that a moderator sees when we delete a post:
So the answer is - it depends. If someone makes a duplicate post, I wouldn't bother notifying them of the deletion. If a post is rude and argumentative and therefore subject to deletion, I may or may not notify - sometimes I'll just leave the reason there in the deletion notice.
If we move or combine threads, I think you automatically receive something, but I prefer to mention that in the thread.
That would be counterproductive to moderation and trying to keep some threads under control. If the offending post is viewable then it could hamper getting things back on topic.