Right now all fields are displayed to public. Since today the Display field to public option is available for every field. The idea behind it is that you can wish to hide the result (formula) value of the calculator and force people if they want to know the result to send it to their email address. That way the percentage of saved results should be higher.
We'll use this option only for formula field, but we added it to every field because you guys always surprise us how you use stuff. We'll be excited to see what other application you can do with it!