Not all media sites require one to be logged in.
However, there are many reasons why a video service might want each user to be individually identifiable by IP.
- Many media items are contractually region-locked
- The same user from too many simultaneous IPs might mean shared credentials, a perceived loss of revenue
- The same user from geographically disparate IPs might also mean shared credentials, even if not simultaneous.
I'm sure there are more.