Viacom sued YouTube when it first got big. Viacom argued that YouTube encouraged copyright infringement and knew about. The safe harbor is voided if the hosting company knew the content was copyrighted or made itself willingly blind of that fact.
So YouTube introduced content id to show that it really was taking copyright serious, and not just using plausible deniability.
Viacom mostly lost the case because there never was much better evidence that YouTube employees knew about individual copyrighted videos on the site.
Google probably keeps the system to avoid new suits and to make business partners happy. A lot of Google content isn't crowdsourced anymore, it's uploaded by big business suppliers who are essentially business partners. I imagine those contracts include content id to be used.
So YouTube introduced content id to show that it really was taking copyright serious, and not just using plausible deniability.
Viacom mostly lost the case because there never was much better evidence that YouTube employees knew about individual copyrighted videos on the site.
Google probably keeps the system to avoid new suits and to make business partners happy. A lot of Google content isn't crowdsourced anymore, it's uploaded by big business suppliers who are essentially business partners. I imagine those contracts include content id to be used.