Swing Sets and Slides

Our neighbors have a really nice swing set for their 4 kids and a slide. It is fun to watch the 3 year old climb up the slide backwards. Why is it kids like to climb up the slide as well as slide down it? I can remember doing the very same thing. I was keeping an eye on her though as she would climb up the slide and then stand on the top and I was hoping she wouldn't lose her balance and fall off. She didn't, fortunately. They all ride their bikes a lot and she has training wheels yet but she's out there with the older kids riding every day.