If for any reasons you make changes to your campaign reflecting a change in the calendar as well, for e.g. Updating it's date/time in case of a reschedule or changing it's description, adding new details about your event, and you wish to "update" all the users who have already received the calendar invite, you will need to goto Send Invite and press the "update" button, and in doing so, credits will be utilized from your credit balance, as these updates are sent out once again as emails to all the users.