A registration ID should be printed, as well as a barcode. If for some reason a particular badge does not scan readily, or an exhibitor does not own a device with a compatible camera, the exhibitor can quickly and easily type the registration ID into their device.
Sometimes exhibitors and/or attendees are uncomfortable when personal space is invaded, as when holding a badge scanning device in close proximity to the badge wearer's body. If a Smartphone or scanner is held too far from or too close to a badge, the scan may fail. If lighting is insufficient or if there is glare off of a badge holder, again, there may be failure to scan properly. For all of these reasons, having registration IDs printed on the badge is a reliable fallback position.
Surprisingly, we find a great many exhibitors prefer to enter an ID, rather than having to scan a barcode. Having both makes everyone happy.