If you’ve ever used categories extensively, you will have noticed that after you save a post, the checked categories appear on top of all the other ones. This can be useful if you have a lot of categories, since you don’t have to scroll.
Unfortunately, this behaviour has a serious side-effect: it breaks the hierarchy. If you have deeply nested categories that don’t make sense out of context, this will completely screw you over.

That’s what this plugin is for: it preserves the category tree at all times. Just activate it and you’re good.

There’s a related plugin that you might also find useful: Category Checklist Expander.