If you have an e-commerce website, then SSL is mandatory for safely processing credit cards. But even if you aren’t processing payments, you should still seriously consider secure HTTP (or HTTPS), especially now that I’m going to show you how to set it up quickly, for free. Let’s get started.
In short, SSL is the “S” in HTTPS. It adds a layer of encryption to HTTP that ensures that the recipient is actually who they claim to be and that only authorized recipients can decrypt the message to see its contents.
Source: Smashing Magazine