Three very much separate parts:
1: Shinya backstory; some interesting world-building, good watch.
2: Fills in some holes, but nothing important, spoilers some of s2 if you watch it in between seasons.
3: Satou backstory; I hoped it would delve into the history of the Ajin and the world a little more, but it's very skippable.
Pretty uneven as a whole and not nearly as good as the main series but still recommended at least once.