(1) I've had a few cases now where people signed up with the emails of others. They didn't get further than sending the activation e-mail, but if they did, you had a fake email in your systems and newsletter. A lot of overhead to get those out again.
(2) Mistyping emails is soo much more common than you might think. If you're unlucky, someone mistypes their email, pays, and then locks themselves out – more work for you.
(3) Due to privacy concerns, it‘s often necessary for the European Union to use a double opt-in procedure. The reason is to make sure that the user really gave his consent. Especially when you already enter some kind of contract. Even trials can be already a contract-based relationship.
(4) It raises the bar for spammers. Requiring a verified email address stops a lot of automated spam crawlers.
(5) If you don't then sensitive information or spam can be sent to someone else's email.
(6) Users can "sign up" with unlimited fake email addresses if you have a free trial (and don't require a social media login).
Credits: Comments of this IndieHacker's post