Not needing root is a game changer.

Another benefit is that "traditional" package managers may litter your system with configuration files and post-install scripts (for the software you are installing and all of its dependencies), some of which can persist even after uninstalling the packages.

Nix prevents that by design.

One less thing to manage and think about, including cleaning up after myself. It may pull dependencies that I don't care about or want exposed e.g that would be picked up by some other thing. apt-get autoremove --purge or pacman -Rsc become a distant memory.

It's fire and forget and I can just focus on what I'm set to do.

Once I'm done with the one shot task, it disappears and could be garbage collected for all I care.

Some tools I did promote to being part of my user env but crossing that bar is a very deliberate thing, some are promoted to systemwide (root) and the bar for that is even higher.

It removes a ton of mental deadweight.