1. Is there a minimal example of this?
e.g. is this how it is supposed to work?

"use client";

const { handleSubmit } = useForm();

<form onSubmit={handleSubmit(serverAction)} />

2. What benefits does it bring for such a case? Or another way to say it, what features are not available in this case?
Maybe progressive enhancement is not available? https://nextjs.org/docs/app/building-your-application/data-fetching/server-actions#invocation