As a Vim/NeoVim user my number one reason is speed. There's a pretty steep learning curve, but it doesn't take long to see noticeable improvements.

Aside from terminal applications generally running faster than GUI ones, there is a tremendous amount of flexibility that it offers when it comes to actual text editing. For example, you learn how to type things like ~~_f(vi(cfoo~~ _f(ci(foo^†^ which goes to the beginning of the line, finds the first open parens, selects everything inside of the parens expression, then replaces that text with "foo". After a while these kinds of inputs become second nature, and you can start using them to construct macros on the fly that can be applied to different places in your code.

One major downside is that it can take some configuration to get working the way you want it, especially if you want an IDE-like environment. NeoVim comes with a built-in LSP interface, which I've been able to get working pretty well for all of the languages that I use the most, but it's still kind of a pain to configure.

I'm sure Emacs is similar, but I've never used it. I don't think many people use Nano unless they need to edit something in a terminal but don't know how to use Vim. On that note, being comfortable with a terminal editor means that you'll have no problem if you're SSH-ing into a server or using the TTY console.

^†^ _f(ci(foo avoids an unnecessary mode change, see comment below