What I love most about the check-in app is that it gives you an accurate record of three very important lists: (1) guests who RSVP'd and actually came to the event, (2) invitees who RSVP'd and didn't show, and (3) guests who just showed up (like walk-ups, guests of guests and surprise VIPs).
Smart event hosts will target these lists with a great email no more than 24 hours after the event. For instance: A “Thanks for coming” email to Group 1, a “Sorry we missed you! email Group 2, and a "Thanks for coming! How did you hear about us?" email to Group 3. And don't forget to add something special in each one – like event photos, coupons, and an offer to subscribe to future events.