1. You can uninstall a thing and it will no longer affect your computer.
2. Your environment will work the same way on your colleague's computer.
3. Your environment will work the same way years down the line.
4. Your environment will probably work the same way years down the line, even after you've applied security updates. If it doesn't, it will probably be clear what's broken/different.
5. You can install many different environments on the same machine, and they will probably all work. On the rare occasions where they don't, it will be because they interact with system services or limited system resources, and it will be clearly defined how that happens.
6. All this is true across most programming languages, system services, and all sorts of tools.
7. You can combine this with direnv (or direnv-mode for Emacs) to make it so that you can just cd into a directory and magically get all the dependencies.
8. You can put the recipe for your environment in Git.
Other than Nix and Guix, I don't know of any tool where these things are true. Docker and Virtual Machines get sort of half way, but tend to rot over time.
But yes, it's difficult to learn and in many cases the usability is poor.
Final bonus point:
9. Nix is so good that it can even make Autotools work correctly.