Reminders help you to stay on top of what's important in your inbox so that nothing slips through the cracks. You can schedule a time to follow up on the emails you send and Reminders will ping you at the top of your inbox with the original email and notes for your next follow-up. You can set them at the time of send on the email, choosing to be reminded if a recipient doesn't reply (you can also set Reminders that come back to you even if a recipient does reply) so that you can follow up, or you can set a Reminder on an incoming email or retroactively on a previously sent email.