We all run events and conferences. And we all collect email addresses of leads.
While I can hear you asking, "Why should I know that in the first place?", there's an email-deliverability-based reason behind it. When you send emails to addresses that have crossed the inactivity-period limit (set by ESPs like Gmail, Zoho Mail, Yahoo, and more), you'll be premeditatively considered a spammer.
Now comes a question: Why do ESPs have the concept of inactivity period in place? The ESPs convert the inactive addresses into spam traps and use them to get hold of spammers.
How double opt-in helps
With double opt-in, only people who click the confirmation link sent to the mailbox become your email contacts. Supposing there's a contact closing in on the inactivity period, s/he rejuvenates their account by engaging with the confirmation link. This, in turn, helps you avoid adding inactive addresses and its consequences.