If your site collects credit card information you are required by the Payment Card Industry (PCI) to have an SSL Certificate. If your site has a login section or sends/receives other private information (street address, phone number, health records, etc.), you should use SSL Certificates to protect that data.
Your customers want to know that you value their security and are serious about protecting their information. More and more customers are becoming savvy online shoppers and reward the brands that they trust with increased business.