It's entertaining for people who aren't perfect at it. Same goes for things like Tic Tac Toe.

Entropy = entertainment

It's quite difficult to play perfectly, even if you study theory and openings for hours.

I could see this being very useful for such applications as synthesizers, mixing apps, DJ consoles, and the like .. heck, any situation where you want the user experience to be channeled in some way. It is often true that swiping a blank-feeling screen, even with pretty lights under it, reduces ones feeling of accuracy. Being able to 'lean on the edges' with your fingertips might make it easier to dodge, bump, scan, and so on, with better accuracy. I'm willing to wager we'll see more of this sort of overlay business soon.