Saturdays are finite. A wedding accepted in March removes that date for everybody else, so a provisional booking without a deposit and a deadline is a date being held hostage by somebody who may not confirm.
Numbers arrive late and change everything. Final counts confirmed a week before determine the order, the staffing and the price, and a system that cannot trace a change back to the quote produces arguments about the invoice.
Staff cost more than the food. Hours, travel and the number of people needed for a service are the variable that decides whether an event was profitable, and estimating them optimistically is the usual mistake.
Equipment travels and disappears. Chafing dishes, linen, crockery and glassware go out to a field and come back short, and a caterer who does not count them out and in replaces stock every season.