In a professional context, you might end up on servers that don't have nano installed, but do have vi. Or if you're helping out a friend on their laptop, they might not have the same software as you. Or if you often end up tinkering with random devices and/or setting up new systems it might be tedious to install the same applications every time.

It's basically an argument for learning the very basics of the most common editors so you have flexibility no matter where you end up. Even when you have the ability to download and install your preferred software, it's still an extra step that might not be desirable for a variety of reasons. But if it's just your own personal device, I see no problem with just installing whatever you prefer and running with it.

EDIT: Personally, I find that I don't end up using those other editors often enough to remember the abstruse commands of tools like vim, so I'm not worried about it. When it does happen, 99% of the time I can just whip out a smartphone and look up the directions for the n-dozenth time.