Because it’s the latest Cool Nerd Thing™ like Arch before it, and Gentoo before that. Most of the people raving about it probably don’t have much use for its features.
The features themselves are very useful for basically any user. Whether they are worth the non-standardness and issues that come with it is another question.
I'm using GNU Guix System which is similar to NixOS and I think I can safely say I do appreciate Guix's features that set it apart from a traditional GNU/Linux distro or package manager.
Declarative orchestration systems have been around for years and have a very real use case when needing to stand up servers in a replicable way. Nix is applying that approach at the system level.
I'm not entirely sold on wanting to put that level of effort into a personal desktop, but I don't think it's at all fair to say that it's not addressing real problems.