[plt-scheme] When is (required ...) required in a formlet?

I just discovered that this formlet

(define login-formlet
  (formlet
   (div "Username:" ,{(to-string (required (text-input))) . => . username} (br)
      "Password:" ,{(to-string (required (password-input))) . => . password})
   `#hash((username . ,username)
          (password . ,password))))

doesn't work if I remove the (required ...) wrapping (text-input) and
(password-input).

I'm fine with that, but I don't really understand what it's doing and,
therefore, when I have to include it. Any insight?

Todd