Availability when utility power isn't (at least theoretically), much lower latency (although everyone else has terrible latency, so kind of meh), desire to spend $60/month for $15 worth of service that the service provider doesn't really want to do anymore anyway. :P

Be aware that in some areas unless you specify a "burglar alarm" or "elevator emergency phone line" or similar, you may end up with a little powered box connected to fibre or similar.

Some have battery backup and so kind of work the same as before ... for awhile.

Bad cellular signal. Kids who aren’t old enough for their own cellphone. Heavy users appreciate the lower latency and audio glitches.

Kids who aren't old enough for their own cellphone is exactly it.

elevator alarm line to connect to legacy POTS wiring going to the panel in an elevator? maybe

An old elevator. If you were to design a new one you would route around the Faraday cage.