By default, the quest log does not preserve the "collapsed" state of quest categories. That is, when you click the "-" on a category header to get them out of the way, they are all shown again the next time you log in. Continuing my tradition of making changes so trivial that nobody else would think to code them, this addon stores those settings across game sessions. That's all. Nothing fancy.
At the start of MoP, I had thought this addon would no longer be required; I was wrong. This has been updated for patch 5.0.5 accordingly. Certain daily quests will still cause some categories to be re-expanded; there's no simple way to avoid that without making this addon take up a lot more memory and CPU cycles than I'm comfortable with.
Only the base API functions are used, so other quest log addons which also use the base API should magically have their collapsed categories saved and restored also. If they do the work themselves then you won't see a difference.