I used Airbnb significantly last year (several 1 month+ stays in 4 different countries across 3 continents) and I found that they usually started with pitching me on other Airbnb instant books in the area; most of them I had already seen and wasn't interested in but I'd guess that this 24hr line will do that first before refunding you and maybe in the situations where there is a sub 20-25% differential (whatever cut airbnb takes these days) they might just true up the difference for a more expensive booking than the one you had to keep you in the airbnb ecosystem. I am a little surprised they aren't also offering a more literal aircover add on with flight insurance for if your flight is delayed or cancelled and you miss part of your stay but maybe that was too out there.