Not a Web guy here: why do people use <script src="foo.js" /> instead of just including the code inline in the script tag?

I understand that as a dev you definitely want your JS in separate files for practical reasons, but couldn't you just have separate .js files and have your framework stick them inline "at runtime"?

PS: Is there any talk with HTTP 2.0 of allowing data like images to be somehow "inlined"? It would then be possible to serve up a 21st century website with a single RTT, right