It's easy to see how this could be useful for somebody though.

Imagine you're a scientist, and you have a large, pure python codebase - let's say for intelligently predicting the number of foos in a bar - and you want to make that available via the browser, in order to visualize the progress of your foo-hunter.

This makes it a trivial task - and means you can use cool web-based visualization tools (D3, etc), and combine with other useful APIs.

I think most scientific apps written in Python rely on native non-pure-python (written in C, Fortran etc...) modules which I'd expect to not work in this context.

If you wanted to go crazy you could use Emscripten but I could easily see pushing the C+Python parts to a server-side web service and then using Pypy.js for the rest.