@Helen796@Jessica and Henry I pondered over this for 2 years- why would they block one day before AND after? But I finally figured this out. It doesn't actually do this. The one day before and the one day after overlap. So you do just end up with 1 night blocked.
Example:
You have a booking for the 3rd-6th, meaning the guest will check in on the 3rd, stay through the night of the 5th, check out on the 6th. The night of the 2nd will be blocked for prep time. The night of the 6th will be blocked as well. But someone can book the 7th. The night of the 6th, which is the "after" block, also becomes the "before" block for the guest who arrives on the 7th.
If they didn't block a night after for prep time, that would mean your calendar would be open for someone to book the 6th, which wouldn't give you that prep time between guests.
Does that make sense?
If Airbnb had stated it "Block one night between reservations" it would be clear, but of course Airbnb seems to be masters of stating things in the most confusing manner possible.