Personally I'd bet on driving general engagement with the platform in some way, but the particular manner is not clear to me.
Well in that case the answer is super simple: The same reason Google provides any other free service, whether it be Maps, Gmail, Photos, Search, Hangouts, Meet, Pay, whatever. The more Google services you use and the more time you spend using them, the more you can be monetized.
Also, the versitility of youtube as a tool leads me to buy youtube premium for $10/mo.
Also, the most professional content creators upload private, then schedule the video public at a time that will get the most exposure by the youtube algorithm. They also pre-prepare multiple thumbnails and swap them out for the first few hours of public exposure. it's a calculus.
Imagine your business is built on Youtube. You want to be able to test things in your videos internally, and upload them prior to a scheduled release date.