Heh, everything. Like Amonika, I use it for die cutting things, which makes it a lot easier to get adhesive only where I need it, but they're really nice for layering and making mats without having to run them through my xyron or use additional adhesive.
This is cardstock that has adhesive on the back of it. You peel off the backing layer and it is sticky. Basically its a big sticker made out of cardstock.
While you can use it just as a "normal" layer on your pages or on cards, you can't use it as a base for the cards because its not double sided. You wouldn't want the backing to show on the inside of the card.
Where it becomes useful (and worth the extra cost) is if you make small punches or diecuts. Rather then try to hold a tiny shape still and apply a bit of glue without getting it everywhere else as well, you just peel off the backing and it is already adhesive. Of course peeling of the backing can be a tiny bit difficult sometimes too. Although I see the usefulness, personally I think there are better ways to spend your money.