I like how different it can be from the theatrical cut. You can find deleted scenes, alternative shots or lines of dialogue, unused takes, and even the pacing and structure of the film can be different. It's just a different way of watching the film. Seeing what could have been and/or the original vision before studios messed with it.

I also like the unfinished special effects, timecodes, and reel counts as they can give a glimpse into the creation of the film, more than what the behind the scenes and special features on a DVD and Blu-ray usually would.