Some problems Nix solves, as concisely as possible:

  - dependency conflicts
  - build reproducibility
  - configuration management
    - configuration drift
  - package management on restricted systems (i.e., installing stuff when you're not root)
  - shared library duplication
  - inconsistent state/broken system after a botched upgrade

This makes it nice for setting up development environments, building containers, and deploying servers.

(A lot of people also enjoy using it for their dotfiles, since it works on macOS as well as across all Linuxes. I personally love it on desktops and development workstations as well.)

It's right there in the heading!

"Declarative, reproducible development environments"