Install Node.js to install n to install Node.js?

I have a problem understanding the use of n. Basically, it is clear that it is a version manager for Node.js such as nvm.

But in contrast to nvm, which is basically a shell script, according to the documentation you are encouraged to use npm to install n:

$ npm install -g n

What I don't get is: For having npm at hand you need to install Node.js. Why would I install Node.js manually to use npm to then be able to install Node.js using n?

To put my question in other words: Why does n suggest installing using npm, if its main purpose is to install Node.js, which includes npm?

Copyright License：
Author:「Golo Roden」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/19451851/install-node-js-to-install-n-to-install-node-js