When editing an Email field you'll see Use as To Address for Email Marketing Lists; on a Phone field, Use as To Address for SMS Marketing Lists. The idea is that the value submitted in that field becomes the contact's email/phone when this form is used as a source for Email or SMS marketing lists ("1 per form allowed", and the current one is shown as "Current: …"). The field must be Mandatory to use this — saving with the toggle on but the field not Mandatory is refused with "This field needs to be mandatory!".

In the current version, note that saving any Email field on the form sets it as that form's Email recipient field (and likewise for Phone/SMS) — the setting isn't limited to only applying when you've just turned the toggle on. So if a form has more than one Email or Phone field, re-saving one of them can silently change which field is used as the marketing-list address; keep this to a single Email field and a single Phone field per form if you rely on it.