Opt-in is the user's permission to recieve your comunication by emails. It is necessary to the success of email marketing campaigns, and also to senders reputation, that will not classify you as a spammer. When you work with a double opt-in, you request a double confirmation to your contact. This way, you have a valid and active email in your list. And it also increases the engagement and click rates, at the same time that protects against fraudulent sign-ups. This way, the chances of your email being rejected by hard bounce greatly decrease.