Credit card payments allow you to take advantage of the two types of customers

From: thesitewizard.com

Impulse buyers. After reading your advertisements and hype on your site, buyers would be all fired up about your product. If they have a means of making a purchase immediately, you've secured that sale. If you only allow cheque payments, the additional time it takes for them to get their cheque book and mail out the cheque may be a deterrence. They may also have second thoughts later.

International customers. Credit card processing payment is a tremendous convenience if your customers are overseas. It automatically takes care of the problems of currency differences as well as the time it takes for a cheque to travel to the vendor. You will lose a large number of overseas customers if cheque payment is the only way you can accept payment.