Why do this? As usual, the Pythons say it best:
For 3 years you YouTubers have been ripping us off, taking tens of thousands of our videos and putting them on YouTube. Now the tables are turned. It's time for us to take matters into our own hands... We've launched our own Monty Python channel on YouTube.
No more of those crap quality videos you've been posting. We're giving you the real thing - HQ videos delivered straight from our vault.
What's more, we're taking our most viewed clips and uploading brand new HQ versions. And what's even more, we're letting you see absolutely everything for free. So there!
They go on to point out that they also supply links to buy their movies and TV shows, but IMHO, the primary benefit of their YouTube channel is building goodwill with their fans. Most people, when confronted with either niceness or meanness, will respond in kind. As shown here:
Here's the thing that paranoid businessmen always ignore: people like to shop. And consumers will buy a DVD of something they've seen before if they like it, because they want the convenience of having it always available to re-watch and the pleasure of having a copy in their home collection.
Like it or not, humans still value artifacts; saying "it's on my hard drive" is not the same as being able to point to the colorful box on your bookshelf. Fans will pay to own a piece of something they love, but they won't love it unless you give them the chance to soak in it, so to speak.