I watch them again, so I can look at them from different perspectives. For example (this is in the audiobook realm, but the point remains), I've listened to HP1-7 looking at it from Harry's point of view, Hermione's, Ron's, Draco's, Voldemort's, Snape's (that was fun... I'd love to see the movies done from Snape's point of view - his sneaking to save Harry, collusion with both Voldemort and Dumbledore, and I'd really love to see the reconciliation with HP after the end of the story. Let him talk to Snape in the portraits, since something must have happened to have his son be named after Snape).I don't get this addiction to TV shows business. Once you've watched the episode / season, what do you do then?
Just a fun mental exercise, I suppose.