Hi Prin,
I imagine this helps in the event of cancellation and/or alterations of bookings. Airbnb holds the burden of issuing any needed refunds. Also, I think it provides a peace of mind for guests in knowing that money isn't already in the hands of the host who actually hasn't provided a service yet. It is similar to booking a hotel in that you reserve but often you aren't charged until you arrive for your stay. As a host, I like it. I don't actually have anyone's money until they are in my prescence and providing them with the accomodation. It just feels better and more honest, in my opinion. Maybe Airbnb will eventually offer a discount feature to guests that allows you to receive the money early but they are not entitled to alter or cancel their stay kind of like prepaid packages for hotels.