Well they already paid for the slots to have them in the first place… as part of contracts with the airport they are either use them or lose them. They give the airlines ability to have flights at certain times.
Not all airports have slots, typically just some of the bigger and busier ones ones, Heathrow, Frankfurt, JFK, etc. If both airports are slot limited you’re looking at two flights that need to occur. Airlines can trade slots around with other airlines.
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