Not everyone who looks at your site will buy something the first time. In fact, 98.5% won’t. However, most people who visit your site without a reason to come back probably won’t do that either.
How do you remind them, then? Well, you could get them to like your Facebook page, but the chances they’ll see any particular status update are negligibly small. You could use an RSS feed, but they’re not that effective, either. Yes, even after twenty years of development in technology, the most effective way is still good old email.
That’s why, in order to retain a significant number of customers, you need to get their email addresses. The form that viewers input their email into is called, straightforwardly enough, the opt-in form.