Performance reasons, also wanted it to work on the web (Flutter web support is fairly recent, and dart/flutter libraries don't automatically work on the web, they have to support the platform).
With the web support I was hoping the since dart can be transpiled to JS Flutter might do something similar. I'll have to look into this, I'm learning Dart due to its ability to run on most platforms.
Performance is pretty good, but as with JavaScript performance, there are performance cliffs depending on how well V8, SpiderMonkey, or JavaScriptCore is able to optimize the code. An audio worklet is pretty standalone and compute intensive, so it seems like a good place to use WebAssembly.