At this point you can probably say choosing node/js is similar to why you would choose Ruby or Python or PHP for your backend, it's another scripting language that offers decent performance, a large ecosystem and good developer productivity. It really boils down to preference in syntax, tooling and ethos ("micro frameworks", "batteries included" or somewhere in between). At this point I'm so comfortable in JS that I can be very productive in Node compared to the ramp up time I would require for Rails, django etc.