This is a pretty poor example for a few reasons, but the idea is that anyone get can a shell with the tool fully installed along with all its dependencies, with a single command:

nix shell 'git+https://tangled.sh/@oppi.li/journal'

It's massive overkill for a shell alias, but for a more complex project it can be very nice.

One area that is especially a massive win is projects that cross multiple ecosystems. So like, C++ project with some Python bindings and a Typescript frontend? Setting up that dev environment is often a nightmare but Nix makes it trivial and highly reproducible.

Sure; I'm typing this comment on a NixOS machine in a browser controlled from a flake I wrote myself - I get using nix and flakes in general. It's just that this particular case seems like such overkill that it actually seems like a weird tradeoff even if you're used to flakes.