My guess would be offloading storage space while adding monetization revenue.
I doubt they're deleting the master copies or the master renders or anything, so the only thing that would be offloaded would be the "consumer" renders. A Blu-ray movie with no additional compression added is between 15-40 gigs.
A consumer like me has a 300 terabyte storage array, presumably Warner Bros has even more than that (and certainly could afford more than that), so it feels like 40 gigs per movie is basically nothing.
This way, they get to ride Youtube's streaming system rather than have to build one out.