The White House Visitors Office is responsible for public tours of the White House, for maintaining a facility where the public can obtain information about the White House, and for other White House events such as the White House Easter Egg Roll, Holiday Open Houses, Spring and Fall Garden tours, State Arrival Ceremonies and other special events.
The White House Visitors Office is needed because not all events are open to the public.
If all events were open to the public then the White House Visitors Office would have been listed as the office responsible for all events for the White House.