Why Must Read What's Different? (Dover Little Activity Books)?

I've gotten several of these Little Dover activity books to take for my four-year-old daughter on airplanes and to restaurants. I think they are great. The "spot the difference" ones are about her favorite, though she does like to have a grown-up sit with her (even though she doesn't need the help). When I ordered my first, I was pleasantly surprised at how thick they were--I was worried they'd be only 4 or 8 pages like the Little Dover sticker sets, but they aren't--they're much more substantial.