It's a similar trade-off to using Terraform to manage a resource, compared to just modifying some cloud resource directly.

You pay a higher upfront cost, but you end up with declarative code which has advantages for maintenance.

With NixOS, rollbacks from changes to system state are trivial; and the whole system is configured from a single starting point, which makes it easy to track down why things are a certain way.

For a personal workstation, it's not necessarily a practical trade-off.

If you manage 3-10+ machines, it's very convincing. 7 steps multiplied by 5 machines, or one edit to my Nix config?

It's even better with flakes. I can keep dozens of projects on-disk, and their dev environment is always versioned + synced with my workstation. I've torched entire weekends trying to get the same workflow out of direnv and pamac/homebrew. On Nix systems, it all lives in one or two git-synced files.