Well, same reason as the Python library (to use another example of something that exists already) - because it’s there. A lot of people know a little Javascript. And also because it’s 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 that’s your bag. Or talks UDP over a network, or spits websockets at a browser. I’m not strictly imagining musical applications here.
I think it’s 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 you’d 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 I’m working within, forcing myself not to means all of a sudden I can spit data from my note-generating app into, I don’t 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 - it’s entirely subjective - but it’s 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; that’s what I mean by ‘enable us to think’. So, personally, in order to express myself: I’d 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 you’ve got 'em.
And whilst I’m not a huge javascript fan, it’s really well suited to this, especially with the rich node library environment that makes integrating with other systems more straightforward.
So: a combination of “because it’s there”, “because we can”, but really the key one for me is “because (subjective) expressiveness”.