It saves bandwidth and leads to faster website loading. A cookie is transported with every request. This is why sites use cookie free domains for static content. If etags were used instead a separate domain wouldn't be necessary.

It's more secure. If identifying session data is not accessible to JavaScript, it makes a site more secure from XSS attacks.

I have been working on a two-factor authentication system. It needs a cookie to keep the user from re-authenticating every time. For one particular user on Chrome, this cookie is getting cleared everyday and they're not sure how to fix it on their side. This could help restore the cookie without requiring needlessly a second factor.

affiliate tracking, analytics, etc.