For me, it's a mix of utility, hobby and learning.
I'm a software developer putting my toes into DevOps. I'm using Raspberry Pi for this and other projects. And I have some songs saved through the years not on Spotify, and videos I would like to make accessible easily.
I use Docker on my Pi for one simple reason: management. While it's true that some services are set and forget, there will be times when you will need to check on a particular service's health, and Docker makes it a little easier by having each service on it's own environment. You can restart every service individually with ease, and even nuke something if needed without wrecking the whole system.
I use Portainer, and everything on Docker inside it.