Why go to the park?

Why do we go to the park? I took my kids to the park today. My four year old dug in the sand. . . we have plenty of that at home. My 16 month old spent almost the entire time climbing in and out and in and out of the stroller, which I would be happy to park in the backyard so she could do it at home. I think I went down the slide more times than they did! So why is it that the kids get silly grins on their faces when I mention the park and they can barely contain their enthusiasm, bouncing in the stroller as they see it up ahead. I don't understand it. Even so, just to make them smile and see them happy, I continue to put on clean clothes and a little lipstick, pack the kids water bottles and goldfish and make the trek to the park. Somehow it's worth it!