1) JavaScript must stay in the box (aka in the browser).
2) JavaScript as a general purpose programming language.
While I can absolutely understand 1), I have had wanted to access the filesystem via JavaScript, just as I do via ruby or python, for local use only. After I googled for a while, they would say that this is not possible unless one uses npm/node. I think this shows that there are use cases here and the "default" JavaScript, aka 1), does not cover these. I do not like JavaScript, but based on my own use cases, I actually favour 2) far more than 1). So from that point of view, being able to access UEFI can also be useful. So why not.