It allows you to use any imagemin plugins you want, which have wrappers for most image compressors out there. And since it runs every "production build" you can commit your unoptimized images to your repo.
In the beta there is a "cache" option, and if an image exists in the cache it will just use that.
However since I use the webpack asset system I can't easily (or safely) check the output to see if it exists (because using the asset system, other plugins could completely change, rename, combine, or delete files after my plugin runs, so trying to detect those files could break things for a lot of users).
1.6 should be promoted to "stable" soon. I only put it in beta since I won't be able to completely test it till Monday. (I also screwed up and merged it into master before I realized I wouldn't be able to do the release untill Monday, so the master Readme has options that won't work in version 1.5)