The assets are already loaded (since you're viewing the bootstrap demo page), so instead of making a request to another server to generate some sort of compressed file for you, that labor is offloaded to the client.
I'm guessing Bootstrap can do it because they know most Bootstrap users are developers with decent PCs but for a more mainstream audience, it would be problematic.
It's 2014 now. Your iphone is 10 times faster than your 1998 pentium 2. Even with the JS vm penalty.
Unless you're zipping 70mb files, there's no chance you're overwhelming anyone's browser.
The usecase for this is the same you'd have for any desktop application: A handy "binary file format" library. Data portability wins.
For most users, bandwidth is in shorter supply than CPU. Especially on mobile (your constrained CPU/IO case), where people are using 3G or worse, which is often even billed by the MB.