Yes, they do serve different purposes but they also share similarities. You could easily write a task queue on top of Faust.
It's important to remember that users had difficulty understanding the concepts behind Celery as well, perhaps it's more approachable now that you're used to it.
Using an asynchronous iterator for processing events enables us to maintain state. It's no longer just a callback that handles a single event, you could do things like "read 10 events at a time", or "wait for events on two separate topics and join them".