I certainly can't speak for others, but my typical use case is projected to be in meetings. Imagine sitting in a room with a bunch of engineers and managers, and someone says something worth checking the math on.

If you're not the lucky dude at the conference room's computer and just happens to have IPython Notebook/LightTable/Excel/`w/e`, then this will be handy. Be the guy who keeps up with the conversation and can check assumptions.

The fact it allows variable names with spaces is just the sweetest syntactic sugar.

It runs a smidge slow on the iPhone, but since that's the tool I have on hand, I'll gladly take a lightweight equation solver like this. (And if the slide-to-change numbers feature gets added, I'll happily buy it again. Twice.)