There are two very basic reasons why I use double opt-in. And why you should too. One is to protect the other people and the other one is to protect yourself.
If you wish to join my email list you must, after filling the simple form on my site, activate your subscription. This is a common practice with email list subscriptions, known as double opt-in. Unlike in single opt-in, in double opt-in just filling the subscription form doesn't add you on an email list. Instead you must first confirm your email address, usually by clicking a special link in a confirmation/activation email. This may sound silly. It would be easier for a subscriber to join a list without having to confirm her subscription. Plus there's always a chance the confirmation email will end up in their spam box and the activation never happens. Sure, that is the possibility.
The first reason, protecting the other people, is very very (very very very) important. Sometimes when I join an email list and there is no confirmation email, I feel like unsubscribing right away. Doesn't the list owner care at all about me at all? Is she more concerned about getting lots of emails to the list than making sure everyone subscribed really wanted join the list? Is she a spammer?
The double opt-in process protects you and other people from getting subscribed to lists without their consent. Like in olden times silly folks would sometimes subscribe their friends to (free) newspapers as a prank. Today email spam is a huge problem for everyone. No person wants any extra unsolicited emails due to someone adding them to a list to mess with them or something. Using double opt-in protects everyone who hasn't had their email account hacked because only the person with access to the inbox can confirm the subscription.
Sometimes people also subscribe to lists by accident. I know, sounds crazy. But imagine if someone didn't quite understand what they were subscribing for. They just thought they would get a free gift or maybe they thought they would sign-up to make comments to your site. The confirmation email can help them to decide if they actually want to subscribe to the list. So another way you just protected everyone. You don't want anyone to be unsure about subscribing to your list. You want them to know they really want to get your emails. Which leads me to the second main reason.
The second reason is a tad more complicated. It is for your protection. For one, when you use double opt-in and don't just autosubscribe people to your list, you can always use that as a sort of a proof that they did subscribe themselves. In time to time someone on your list will forget they subscribed to it in the first place. I have forgot that, especially when there has been a longer (than a couple of weeks) pause between the emails. So if you use double opt-in, at least you can always say that they didn't only fill their email on your site but also did click the confirmation link in the confirmation email. Plus spamming is illegal in many places and there are many other laws about sending unsolicited emails. You don't want to get in legal trouble with your emails, do you?
In case you provide some sort of an incentive, such as a gift (like my Indie Online Marketing Workbook) some people may think they will only get the gift and subscribe for it. So double opt-in will also help to keep people who aren't really interested in your emails out of your list. Some email marketing services offer unsubscribers a way to complain about the list when unsubscribing. Say that they never subscribed for it. That can easily get you in trouble with your email marketing service and lead into losing your list. The other thing people like this do is mark your emails as spam. Which will give you a bad reputation. For example if lots of people mark your emails as spam in Gmail, the service will begin to mark your emails as spam automatically. Your domain can also end up in one or more email blacklists, which will cause your emails not to be delivered in the future. You really don't want that, do you?
Double opt-in also can protect your email list from spambots. Spambots subscribe to email lists for various and frankly somewhat unknown reasons (maybe they are lonely and want your emails to warm their virtual existence). In parts it is to harvest your email for spam usage. But what surprises me more are those spambots that go through the trouble of bypassing double opt-in. It happens. It was part of the reason why I switched from MailChimp to Mad Mimi. My MailChimp list was getting filled with spambot emails, even though MailChimp forces double opt-in. After a quick search I found out this is still a problem with MailChimp. So okay, it is not perfect, but it does help. At least at Mad Mimi I very rarely see spam email addresses getting through the double opt-in process.
Now you are asking me how did I know those emails were spam. With a little wit you can recognise the spambot email addresses on your list right away. For example the name a spambot provides (if your form/list requires providing a name) is a often a combination of a name and a number sequence. Some spambots use temporary email addresses, such as guerillamail.com, mailinator.com, mailnesia.com and thrma.com. Also mail.ru email addresses are nothing but spam.
If you aren't that concerned about spammers getting to your list, consider the fact that they will be filling your email address and sending quota. Email marketing services bill you depending on the amount of subscribers but also the amount of emails you send per month. So if there are loads of spambots on your list, you will end up sooner or later paying extra for them. Removing them from the list can be a painful job, depending on how your email marketing service works. Even if you use your own email marketing system, for example a self-maintained email list plugin or module within your content management system or blogging platform, the extra emails will slow your email sending time down. Keeping the spammers off your list is an excellent idea for so many reasons.
For these reasons I use double opt-in for my email list and I really recommend you to do so too. It is not only to protect your email list but also to protect the emails of others. And that is a cool thing to do.