Confirmed Opt-In is used to prevent spam and ensure that only people who specifically request your information are subscribed to your list.
It prevents people from subscribing false or malicious addresses to your list
It gives you proof that all subscribers did request your information, and protects you from false spam complaints.
As an ISP standard for managing mailing lists, it ensures the best possible deliverability for your messages. Consider the following from Yahoo, possibly the most popular free web based email service:
[U]se confirmed, opt-in email lists. To do this, after you receive a subscription request, send a confirmation email to that address which requires some affirmative action before that email address is added to the mailing list. Since only the true owner of that email address can respond, you will know that the true owner has truly intended to subscribe and that the address is valid.
Source: Yahoo Mail Help