Reproducible dev environment. Easier to reproduce some bugs for fixing. More certainty that it doesn't just work on your laptop because you have an undocumented dependency installed. Easier to test the setup process on a clean machine and vary things about the machine setup. To test what happens if you have Python available system-wide vs. if you don't. More precise development history since you have docker-compose.yml under Git, making "time travel" easier.

There's all kinds of little benefits that don't seem that important until you have use for them. Of course Guix and Nix go closer to being actually reproducible, but Docker is better than nothing.