When submitting data to a server, callers need a callback that will
get invoked after all the individual form fields are saved. If they
have a button that submits the form, they could just do this logic
in the click handler for the button (save the form, then submit to
the server), but if they have more ways than one to submit the form
(i.e. hitting enter while in a text form field), then it becomes
more convoluted and calls for a unified callback that will get
notified when the form is submitted.