Because that way you can use it wherever something accepts WASM. In particular, as mentioned in the linked article, Javy started its life as a way for you to submit code to Shopify Functions in JavaScript, as Shopify Functions lets you submit code as WASM so that you can program in whatever language you prefer.
Yeah, when I was a kid and Doom had first come out, I got scared to death when I walked around a corner and ran into my first pinky; it was horrifying!