I was recently scrolling through my "Favorites" playlist on YouTube - which dates back to when I first created my account nearly two decades ago. A surprising number of those videos are no longer watchable on YouTube. These aren't even controversial things - just random things that may have been copyright-striked out of existence, pulled by the original uploader for whatever reason, vanished when the uploader deleted their account, pulled by a new owner of channel after some merger/acquisition, etc. So one simple reason is to preserve access to valuable video content.
