You can download the movies you purchase on Apple's iTunes Store. The content is tied to your AppleID, though, so you can't simply move the file around and duplicating it doesn't work.

There's probably a way to remove the protections, but I never checked.

But, on that note, you might want to check Jeff Geerling's channel on YouTube. He posted a video last week about owning your media and he explains how he started ripping discs to have his movies in the computer - and even stream it, though only inside his own network.