For single-user single-device scenarios, you probably won't find Nix much better than brew or apt. Declarative infrastructure isn't very necessary when you're the only person running your software. With multiple devices, you at least want a way to automate your build process. Bash scripts and Makefiles work fine, but they both fail silently and won't work consistently across devices, OSes or even architectures.

Nix smooths that out, which is great for a single user with multiple devices but even better on teams. Instead of coordinating devices individually, you can update Nix environments and push them out to everyone at once. The build environment is self-contained, isolated and updated silently alongside the rest of your repo. On larger teams, that saves a lot of configuration headache.