Any reputable business will require a deposit from you to secure their services. They are used to cover the labor and time involved in preparing for your event, regardless of whether your wedding takes place or not. As many vendors reserve the weekend exclusively for you, and thereby turn away other prospective customers, it is only fair to show your goodwill by providing the deposits they require.
In the event you need to postpone or cancel your wedding, do not expect to have your deposit refunded. In some countries state and local laws say that a deposit given to secure a time, service or product, is not refundable. The only reason a vendor would be required to refund your deposit, is if they are unable to provide the contracted product or service, or not to the standards agreed upon. If you are lucky, some may, for the sake of good customer service, refund a deposit in certain cases, so asking wouldn't do any harm, but it's at their discretion.