Why adding version number to CSS file path?

Question

I noticed some websites put the version numbers (especially) in the CSS file path. For example:

Answer 1

My knowledge is pretty much out of date regarding websites, but the variable stored in the 'href' argument is received by the browser through HTTP. Using the usual tricks in URL-rewriting you could actually have an arbitrary script that produces CSS-output when called. That output can differ, depending on the argument.