Why would one implement this instead of using Android's builtin widgets? Android apps have always felt better to me when they use the UI recommended in the Android style guide.
Because the built-in resources are very Google-brand looking. Big apps have their own style. They'll follow UI norms, but don't need to look like Google made the app.
I guess the only real use is for the icon font, but people were doing that already and it's best practice to generate one with just the symbols you need.
This could be a good use case for sites that have a "mobile version" but don't want to/need to mimic the full android look but get close.
EDIT: I am definitely wrong on this (didn't read the docs careful enough). Thanks for the correction on this everyone.
This isn't for mobile HTML5 development.