Sure if your project is almost exclusively CPU intensive then use something else. But sometimes you have a project that already is in JavaScript and where JavaScript mostly makes sense, but you have one or two task that are small but fairly CPU intensive. Then knowing how to write fast JavaScript is a pretty good idea since JavaScript can be pretty fast these days and the overhead (both in terms of runtime and developer time) of calling out to second language isn't always trivial and it's nice if you can avoid it.

Secondly (and not entirely relevant to this) people are doing more and more 'clever' CPU-intensive things client side these days where JavaScript is your only choice. So understanding the performance characteristics of different JavaScript implementations can definitely come in handy there.