Just because it was fun. And I could.
But really, part of it isn't because it's so easy (though it is). Part of it is that you give up a bit of control. When I use Photoshop Elements or Lightroom, there are almost infinite degrees of adjustment you can make to a photograph. I doubt I use even 5% of what these programs are capable of. Shakeit - at least the version on my iTouch, gives you one choice - do it, or not. There's an unpredictability to how it crops, and the colour treatment, which looks a bit lomo-ish to me, with the garish colours and blown out highlights, can't be adjusted at all. I understand the version on the iPhone gives you filters and other options, but on the iTouch, it's just the one thing. Shake, take what you get, and upload. Even lying in bed! That's cool.