Double Opt-in is the only sure fire way to ensure that people who fill out your sign-up forms actually own that email address because they then have to go to that inbox and click a link to confirm the sign-up.
If the email address just goes straight onto your list without any confirmation there is nothing to stop email addresses of people who don't want your email getting on your list and subsequently getting spammed. They in turn will mark your emails as spam and your reputation and deliverability will plummet.
However, many brands refuse to do it because they see it as a barrier to entry for people and won't risk people not clicking on that confirmation email.
I've written a blog post for Pure360 guiding through the optimum way of using double opt-in, getting people to happily click to confirm and I explain why the people who don't aren't worth the risk to your deliverability - with a little example of how a brand suffered the consequences...read on