I have several use cases, a big one being that it gives me an alternate storage medium for backing up home photos and videos. Obviously there's caveats on how long BD-Rs last (although M-discs should outlast me) and the issue of needing a player in future, but it gives me more peace of mind knowing that I can backup these sorts of things to different storage types (external hard drives are all well and good until they're corrupted by power issues or user error, or you want to keep a copy at a relative's place and it's a multi-hour trip... with optical media you can just keep adding discs to the offsite backup as needed and update the external HDD less frequently).

The other major use case I have has already been mentioned - backing up Blurays that I've bought (or, in the case of a few shows I like, being able to compare the DVD vs Bluray frame by frame).