You just set up a new Linux distribution on your computer, all nice and clean. But, over time you fix problems and enhance the system, and eventually you do not know how you got to the point you got to, and the system is honestly kind of a mess.
This is one of the big problems NixOS seeks to solve, through use of a single and powerful configuration system, you can keep track of every thing, from the entire system to each user. The goal is to be reproducible.
But, obviously, like everything else, it has a number of shortcomings, like the fact there is hardly any graphical tools for installation and package management, as well as not that much configuration options for KDE and Gnome.
Overall though, it has much potential and I personally have found it useful for easy control and management.