If you fly from XXX-YYY-ZZZ as 2 flights with 2 return flights as XXX-YYY, YYY-ZZZ, ZZZ-YYY, and YYY-XXX you will have 4 separate demand pools. If you fly from XXX-ZZZ via YYY, you'll have only 1 flight with 1 return flight as XXX-ZZZ and ZZZ-XXX and only 2 demand pools.
This is useful for long flights. For example, I'm flying from San Francisco to Dubai in a game with Lockheed Tri-Star with a range of only 3700 miles. I can barely make it to Europe in 3700 miles, much less Dubai. Therefore, I fly to Dubai with a refueling stop in Iceland (just under 3700 miles) and I continue on to my destination. A lot of passengers want to fly from San Fran to Dubai, but there aren't a lot of passengers who want to fly from San Fran to Iceland or Iceland to Dubai, so I'm harnessing the demand of SFO-DXB instead of Iceland's less than robust demand.