The last mile of flight time is the most expensive because the jet fuel has to be carried all the way from the starting point. By breaking a trip into intermediate hops, you get to trade “expensive” last miles for cheaper ones.

For it to make economic sense it just needs to exceed the extra cost of time spent at the intermediate port. And even that can offset by adding cargo for that intermediate destination to replace the weight savings of not carrying that “last mile” jet fuel.