If you've ever driven along some of the interstate highways in the Midwestern to Western states, you've probably realized two things.
1) South Dakota / North Dakota / Iowa / etc is REALLY boring.
2) Why are there so many billboards saying "Wall Drug"?
If you actually get off the highway at the exit for Wall, SD, you'll run into Wall Drug, the store that paid for all those billboards along the previous 300 miles of highway.
It's an actual store / self perpetuating tourist attraction, right up there with the world's biggest ball of string, etc. I must say, if you're passing by, you might as well visit it. After all, what else were you going to do in South Dakota?