These days with graphql, or complex microservices architectures, you could have multiple hops to fulfil l the original request.

Flask sync will hold that thread hostage until the request is done. Where async with properly used async libs will allow other requests to process.

We often have medium sized reports take seconds. That is a lot of time to wait. And would just end up bloating your service scaling to handle more connections.

Any service with decently long lived network requests will benefit from event loop handled scheduling.