Well, same reason as the Python library (to use another example of something that exists already) - because itās there. A lot of people know a little Javascript. And also because itās actually a nice fit for doing realtime programming: it handles asynchronicity well, thanks to first-class functions and promises and the like.
So to pull focus a little wider: sure, you can make an application that emits MIDI. Or OSC, if thatās your bag. Or talks UDP over a network, or spits websockets at a browser. Iām not strictly imagining musical applications here.
I think itās likely that your node app is not the only part of the chain. But you could also do things that go beyond that, given a grid is just some buttons and LEDs; you could make a 128-key clipboard for your text editors; something that visualises data sent to it from the computer; honestly, whatever youād like.
(I also think node not being the only part of the chain is a good thing: rather than having to generate audio in whatever platform Iām working within, forcing myself not to means all of a sudden I can spit data from my note-generating app into, I donāt know, an Ableton Live Instrument Rack that I really like, and which is a great environment for designing sound in for me).
From my perspective, the advantage is that my brain does not work like Max or pd. At all; I find them unintuitive and hard to think in. This is not an absolute - itās entirely subjective - but itās also true. Programming languages are tools for humans, and we cleave to ones that allow us to express ourselves, that enable us to think. Sometimes, a language maps to our mental model; sometimes, our model has been shaped by langauges; thatās what I mean by āenable us to thinkā. So, personally, in order to express myself: Iād like functions and variables, ideally that I can pass around; control structures; a text-based language; classes, sometimes; a language I can write mainly in vim and a terminal; a fast REPL loop; tests, if youāve got 'em.
And whilst Iām not a huge javascript fan, itās really well suited to this, especially with the rich node library environment that makes integrating with other systems more straightforward.
So: a combination of ābecause itās thereā, ābecause we canā, but really the key one for me is ābecause (subjective) expressivenessā.