You can also scan a paper boarding pass. Whether from a kiosk or printed out. Not everyone uses mobile boarding passes. Some airports also don't have good (or

You can also scan a paper boarding pass. Whether from a kiosk or printed out. Not everyone uses mobile boarding passes. Some airports also don't have good (or any, although that's increasingly rarer) free wifi and not everyone has roaming. Some people don't have smart phones at all.