By design this feature will lookup a contact record via e-mail. If the contact exists, it will update "Optional fields without a value".
User story to change this so that all except the lookup field get updated without regard to whether the field has a value:
1. Company attends yearly trade show. The usual suspects show up. Their attendee information is entered somewhere that is available to Zapier. They enter their current information for address, etc. This would be an ideal time to get Act updated with this information. What happens often is that duplicate contact records are created and no one is really sure which record is the correct one.
2. It would also be cool to be able to update a multi-select field. For example, Referred By has multi select field for the year of the conference (Acc Conference 2016).