The tiny electronic devices remain a vital means of communication in some areas - such as health care and emergency services, thanks to their durability and long battery life
People that work on-call do this, especially in tech or security.
I'm considering making the switch because my paging calls are from a random set of phone numbers, so I cannot attach a specific ringtone to them. After a few horrible pages, you start to associate your phone going off as a world-ending experience, when it's just your wife calling to ask if you want her to pick something up for you from the shop. A separate device that disassociates my phone from pain would be nice.
Opsgenie and PagerDuty let you add them as contacts from within the app and it manages the rotating numbers for you so you can keep using a specific ringtone for them. This is also how they can override DND so you can go back to muting your phone at night and know that pages will still come in.