Due to its functional nature, it can just download a binary package with the same hash if its available and it'll get the very same package as you'd build locally (to the last bit that is).

Since packages are stored in the /nix directory, it won't overlap with anything you'd install in /usr/local or wherever else.

Every time your profile changes, you get a new generation of your profile and older generations are kept around, so you can easily (and atomically) revert to older version of your profile.