Say your flight gets cancelled, who is going to book a replacement flight? You are, not them. If you use a travel professional that travel professional will do it. If something goes wrong on your trip, if the room you booked is not like what you thought it would be, who is going to make it right? A travel professional will also check constantly for price drops before final payment and whether a new promotion offered would be more beneficial than what was booked with a deposit. All these things can be addressed before final payment.