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

If you set caches to expire far in the future adding ?v=2 will let the server know this is a new file but you won't need to give it a unique name (saving you a global search and replace)

HTM5 boilerplate also includes it in their project.

Check this video also: HTML5 Boilerplate Walkthrough.