Quotes allow a guest to follow the standard checkout process. Quote acceptance ensures you've got a signed renter agreement, payment is taken, and emails are scheduled.
If a guest has already booked on a different site, like through AirBnb or HomeAway, then its a good idea to create a booking (or you can create bookings automatically via Calendar Import). Or if you are importing bookings for historical purposes (although we have a faster way to do that with our Booking Excel import, available in the Tools menu).
If a guest is checking out through you, you should send them a quote and have them complete the checkout process using the link in the quote email.