Confused about pruning

I have done searches on pruning, and some prune to delete and some prune to their basket.

I understand some reasons to prune to basket, to act as an archive, also to not effect the members post counts. But what is the use of pruning it to the basket in the first place? If you are going to keep it, why not keep it where it was? I mean, isn't the purpose of pruning in the first place, to clean things up by making more room or space on your board? So that you don't go over the post limits?

The reason why I ask this is because I would like to maintain my members post counts, but I plan to put in several chain word games in my forum, which I know can take up alot of space. So I don't want to run out of space, so I want to trim off some of the posts. But from what I understand about auto-pruning is that it takes the entire topic and not the individual posts? Isn't that correct?

If you enable pruning in the basket it will automatically delete those topics(and the members will lose there number of posts which they made in that topic).
To check more things about pruning go to admin panel=>Forum=>Management=>Categories and forums/ Choose a forum from there and scroll down the page until you find the auto pruning options. This way you can choose the time you want to keep topics that don't have a reply.