I'm using my own examples here, one of the projects I'm apart of distributes personal videos over HTML5. Having these just stored on a file system is vulnerable in a way that a sysadmin could view the video or if the file system is compromised [the exploit could reveal the video]. Those are what I could think of off the top of my head.

Having the video encrypted (since Netflix is pushing for it anyways, why not use it to my benefit), allows me to provide the keys directly to the content owner for decryption.

Would you do this by creating your own EME in a cross-browser manner?

I guess the built in support for basic encryption would work here (the mandatory profile).

You could even deliver the symmetric key out-of-band with asymmetric encryption.