I've done some browser applications using kotlin-js. One of the nice things with kotlin-js is that you can use co-routines. That's a nice upgrade over of promises or even async await in typescript. And you can easily await javascript promises in kotlin (extension function) or deal with javascript code that expects a promise. Co-routines basically gives you structured concurrency, a bit more sane error handling with this, and a few more things. Javascript and typescript are a bit bare bones with this.

And of course in a browser, everything is basically happening asynchronously. So, there are a lot of places where co-routines can be useful. And you can just spin up stuff with launch {} as well and have things running in the background. Nice with web sockets for example.