Within the form builder, you can either create new fields, or select from existing ones. This can be very benefitial when your site contains multiple forms, and you find yourself creating the same field over and over again. An email address field would be a great example of this - used in almost every typical form.
To add an existing field, access the form builder, by editing any form. On the right-hand side, there will be a "+ Add existing field" button, above the draggable fields. This will open a modal window allowing you to select from your existing fields.
Field already included in the form will be greyed out and un-selectable. You can search for your fields, and filter by any other forms you already have. You can also click multiple fields at the same time.
Currently, Repeater fields cannot be added through the existing fields functionality.
Formie provides additional options for adding an existing field.
Selecting this option when adding an existing field will copy the field(s) original settings, and create a new one. Any edits to this new field will be completely independant of the original field.
In contrast to adding an existing field as a new field, you can add existing fields as a Synced Fields. This maintains a reference to the original field, and any changes to either field will by synchronized. See Synced Fields for more.
Available for Craft 3. Get it from the plugin store.