Some restaurants have a policy stating that they require the organizer to be there in advance of the fundraiser, and others ask that the organizer be present for the entire fundraiser. You can see if the restaurant you are hosting with requires this on the Meal Management Page for your event (get to this by logging in to your GroupRaise dashboard and clicking on the event in question).
Restaurants sometimes ask that organizers do this in order to direct traffic, give instructions to supporters to make sure the sales are properly accounted, and prevent attendees from soliciting patrons that are already in the restaurant. However, many restaurants will not actually require you to be present as long as you reach out to them directly before the event date to let them know.
COVID-19 UPDATE: Due to on-premises restricted put in place at many restaurant establishments as a result of COVID-19, it is important to reach out to the restaurant if you see this policy listed for your event as the restaurant may no longer require you to be present.