A very fair question asked by many clients!
A deposit is not only to secure your booking with the vendor, but also to secure the vendor's business with you. Confusing, yes, but follow me here.
Photographers and reception sites can logically only book one wedding per day. Cake creators, caterers and many florists can only book a certain number of weddings per day, and some of these limit it to one per day. When a bride books that date, the vendor closes that date to any other bride/booking. This means we may turn down business from future inquiries for that date.
(I personally turned down over 15 brides for the date of 7/7/07 because it was booked, and I turned down over 25 brides for the popular 6-7-08 date.)
Should something happen and you change your mind, move your date, want to use another vendor, and you cancel your booking, especially close to the date, it creates a financial hardship for the vendor. As you know, not too many brides book their caterer, photographer, etc., 3 or 4 weeks before the event.
This means the vendor has no opportunity to re-book that date. The vendor has already turned down business that could have been booked except for the fact that he/she closed that date just for you. The non-refundable deposit covers the lost opportunities and the costs incurred by the vendor based on your commitment.
Moving the date is many times viewed as a cancellation. Why? A bride books 11-7-09 with me. I turn down 4 brides for this date because I've made a commitment to the booked bride. Bride decides to move her date to January, after the holidays. I have now lost those 4 potential brides because I've turned them down.
I've had a number of discussions in which I'm asked "What costs? The photographer hasn't taken any pictures yet!" You're right. And the caterer probably hasn't bought any food and the florist hasn't purchased the flowers. But these are not the only costs associated with running a business that can help you have a great event. All of these vendors require special (expensive) equipment, rent, insurance, staffing to answer the phone when you called to book the event, internet access to enable you to see our website as you planned your event, the time already spent with the client during the consultation and follow-up, cost of paper, folders and filing cabinets to create and store the client's paperwork, and much more. These costs need to be covered and overhead has to be met.
When we close that date to any other business based on a client's commitment, then we have lost the opportunity to see any income for that date that would cover this overhead when the client decides to cancel.
A deposit protects the client and assures the client that the vendor will be there. A deposit also protects the vendor when the vendor turns down other clients, so the vendor can be there for you on that date.
So when talking to vendors, ask about their cancellation and refund policies. This is especially important if the bride/groom is active military and unscheduled deployment is a real possibility.
Knowing what the rules are in advance will help reduce some of the pre-wedding stress!