I taught Kindergarten, so the kids would use them to save any "reward" type stickers or whatever.
you could use them for other things though too, like practicing letters/writing...
you could have a set of alpha stickers in each book and then they could have a fun activity putting words together or making patterns, etc.
Another idea that is perfect for sticker books is to get a cheapy cheap photo album - I got one that had room for 32 photos at the dollar store. Just plastic sleaves that the photos go in, right? You could design a front and back cover if you like, to replace the ones that come with the book and there you go! Instant sticker book. In my gallery there is a hybrid recipe album made out of one if you're not sure what kind of book I'm talking about...