I'm guessing that it's so that when you search for a long duration of stay, many of the listings will be unavailable but with split stays they can make it easie

I'm guessing that it's so that when you search for a long duration of stay, many of the listings will be unavailable but with split stays they can make it easier to figure out how to plan a longer stay, so you don't have to look at the availability calendars of a bunch of different listings.