For me the metadata is a big part of it. I have a large physical media collection so I could just use the discs, but putting it all into plex has made it so much easier to browse and watch. For example I can pause and skim the cast/crew filmographies right in the plex UI, including being able to see what I already have and investigate the stuff I've never heard of for possible future purchases. When watching several movies in a row, after finishing one I can follow the automatically-populated trail of related/similar movies until I find something that fits my mood.