Everyone over the age of 6 in the cabin must purchase.
It includes priority seating at shows, wifi for one device for each person for the duration of the voyage, priority at check in, priority off at ports, lunch on embarkation in the MDR from the Chops lunch menu, and the service of dropping your carryons on a luggage cart from which they will deliver them to your cabin for you.
The financial equation is usually: if the cost per person is not much more than the cost of the wifi alone, it's a decent value. Otherwise, unless you are a "priority" sort of person, not so much.