Theme parks sell products that move a visitor toward the front of a queue. The practice is not simply an extra charge, and it follows from the fact that a ride's capacity cannot be increased on the day.
Ride capacity is fixed and cannot flex with demand
A ride dispatches a set number of vehicles per hour, each holding a set number of riders. That figure is determined by the engineering of the system.
Demand, by contrast, varies enormously by season, day and hour.
Since supply cannot move, the excess demand becomes a queue, and a queue is a way of allocating a scarce resource by time rather than by price.
Queueing time is a cost with no revenue attached
A visitor standing in line is spending something valuable and the park receives nothing for it.
Worse, a person in a queue is not in a shop or a restaurant, which is where a large share of park revenue is earned.
Converting some of that waiting into a paid product captures value that was previously being burned.
Different visitors value time differently
A family visiting once in a decade, with a fixed number of days and a long journey behind them, values ride access highly.
A local annual passholder who can return next weekend does not, and will happily wait or come back another time.
Charging for priority separates those groups, and each pays according to what the visit is worth to them rather than paying the same admission price.
The system redistributes waiting rather than removing it
Because capacity is unchanged, every person moved forward moves someone else back.
Parks manage this by limiting how many priority slots exist per ride and per hour, since a queue that visibly stops moving generates more resentment than a slow one.
Virtual queueing, where a visitor holds a place without standing in it, addresses the same problem by freeing people to spend money elsewhere while they wait.
Pricing has moved toward continuous adjustment
Parks increasingly vary the price of priority access by day and by attraction, raising it when demand is high.
Ticket prices themselves move the same way, which spreads attendance across the calendar and reduces the peaks that create the worst queues.
The whole apparatus is an attempt to manage a fixed capacity against variable demand, using price where physical expansion is impossible.