The difference? I don't pay for access to a restaurant just to find out I can't eat there. If I walk into a Fleming's Steakhouse and can't be seated in an appropriate time, I can leave without losing a day's admission to a theme park.Think about it like reservations for a restaurant. Instead of going to restaurant to restaurant looking for the shortest wait time, or even sitting there waiting 2-3 hours for your table to be ready, doesn't it make sense to call in, or make reservations online? If one isn't available that day, you go somewhere else. If its a 2-3 hour wait, you put your name in and show up in 2-3 hours.
Why isn't it the same for rides? What practical purpose does it serve to have to wait in a caged in area and walk every now and then for hours? Why couldn't the waiting be anywhere? Mainly, you know going in beforehand if you'll be getting on or not. Stand-by lines just make little practical sense over a reservation system.
The original concept of FP (move guests out of attraction lines so they could spend money in restaurants and gift shops) would work only if WDW could somehow exclude people with FP+ reservations from other attraction lines. Without that restriction, guests are double-dipping and taking up twice the ride queue space they're nominally entitled to by their admission tickets. WDW got away with it until (a) the advent of FP+, and (b) the latter part of the 20-teens, when the increase in visitors far outstripped their static inventory of attractions. Sadly, FP+ turned into a "minimum ride rationing system" instead of the shopping and dining boost it was designed to be.
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