This is just a matter of preference, but an SSL Certificate gives you and your customers a secure browsing experience. Especially if you're collecting credit information, or if people create usernames and passwords on your site. Also protecting you and your customers/members/visitors from phishing.
The disadvantages are that it costs a lot and it affects website performance, an SSL Certificate takes more server resources. But this is something that happens when you get a lot of traffic on your site, however it can also be fixed easily by adding more resources to your server.
Google also loves secured websites, most people will feel safe when browsing your website, you can also embed your website on a facebook tab and get dead cheap cost per click as well.
Whether it's necessary or not is up to you, the type of business you're running and your budget, among other factors.