I read a thing once (I really should find it) about DevOps and notifications (I think) where the author said something like:
Email is a terrible notification system. The only thing worse than email is everything else.
The point was like… you should probably have phone or SMS-based alerting systems for DevOps problems, but that can have all sorts of problems. A systems failure could affect the delivery of those messages. A user might have a billing problem with their phone carrier or whatever and not get them. Or be on silent mode. Those are just edge cases, but every kind of notification can have them. Push notifications can have permissions problems. You could send a Slack message but maybe the user doen’t have Slack open. Whatever.
It’s not like email really solves any of those edge cases specifically, it’s just the email is so damn reliable that it makes for a very sensible notification, at least as a default option.
So if you’re building any sort of application that needs to send out messages or alerts of any kind, having email as the default option just makes good sense. Omitting it entirely would be bonkers.
Email is the least common denominator. Or is it the greatest common multiple? I feel like the later makes more sense but the former is a more common saying.