Because that will facilitate your reservations! In particular on two points:
When you are on your Bnbsitter services calendar, your reservations airbnb are highlighted by red headbands. This allows you to manage more effectively your Bnbsitter reservations according to yourAirbnb rents:
ATTENTION: this synchronization does not automate the reservations at Bnbsitter. You have to make yourself the reservations.
With the aim of making you save time, the synchronization of your calendar fill automatically your guests contact. Exit this boring task, the synchronization takes care of everything.