not even necessarily in web servers. If one writes a frontend proxy server which needs to work on > 10k concurrent connections it's likely the way to go. But for an application server which sees much less concurrency it might not matter that much.

Web servers and web browsers, which is a lot of todays dev work.

Why would a web browser need async? It can use threads.

Well nothing needs async, you can use threads anywhere. But it is the predominant model for JS

It's the model in JS because JS does not really support threading (web workers are more like separate processes) so it's the only way to do it.

The actual browser doesn't need async.

When I say "need" I mean that it provides a decent performance increase.