The single killer feature that convinced me to move to NixOS is the ability to very easily keep separate development environments separate. For instance, if you're working on multiple dev projects that have different minimum requirements, and you want to ensure that (for instance) you don't accidentally use features from after boost 1.61 for project A, because that's the stated requirement, but you need features from boost 1.75 in project B.

In a normal distribution, in order to set up an environment that has the proper version for project A you'd need to set up a chroot, a virtual machine, a complicated set of environment variables in a bespoke script with custom installation paths that you need to set up manually and remember to source, or just install a newer version of boost and rely on continuous integration to catch it if you screw up.

In NixOS, you can set up different shells which all reference the exact correct version of the libraries required for every project, you can have them installed simultaneously and without conflicts, and there's even a shell hooking program that will automatically load and unload this configuration when you change directories into and out of the project folder. It makes managing many different projects much easier. It's like a better version of venv, but for everything.