He mentions he'd like to use it to get windowing environments compiled over to JS for use in the browser. I scoffed until playing around with his real live python 2.7 running on javascript in Chrome. YOW! That's just super cool.
It looks like you can use it as essentially an LLVM backend, meaning that any compiler/interpreter that uses LLVM can compile into Javascript (for use in the browser).
imagine a hugely useful c library being compiled into javascript for use in a browser env and even nodejs.
on the latter example: you could also just compile that hugely useful c library as a nodejs plugin directly.