Apple uses client certificates for their APNS API (push notifications). Both to verify that you can actually send push, and to identify which app and environmen

Apple uses client certificates for their APNS API (push notifications). Both to verify that you can actually send push, and to identify which app and environment (dev or production) you're sending push for. I have also used client certificates for a number of computer-to-computer systems. So client certificates are being used, even though people aren't using them in their browsers.