If you have a business, your customers need pay for purchased goods through online. To enable you to accept the plastic (credit/debit) cards you will require a merchant account or you can opt to use a third party processing service such a PayPal.The advantage of using a merchant account is that:
1) The money paid by the customer is deposited into your bank account in two to three working days whereas when you use a third part payment gateway it nearly takes longer time comparatively.
2) Merchant accounts are more cost effective like the one from http://www.xnynz.com/ecommerce/merchant-services.aspx?ci=1804&prog_id=456858
and have the option of paying a monthly fee or per transaction fee whereas third party payment gateway doesn't allow a monthly fee and their transaction is also much higher.
3) If you have a merchant account, customers never leave your website when they are making a purchase and when they get their credit card bill, the name of your company will show up on the bill thereby increasing the customer's satisfaction and attention.
4)The most important is the customer need not have a Paypal account and is liberal to pay through any mode(bank,credit or debit).