Most webapps have a need for transactional messaging of some sort. But even putting that aside you’re missing TOS support. Every TOS is going to have conditions that define scenarios where you need to communicate with users. Including changes to the terms of service ;-). Or for breaches of data, you’ll want/contractually need to communicate that to your users.

So yeah you could build different options for all of these things, and maybe snailmail users of these TOS occasions. But the system of doing this over email is so standardized that the possible friction of one-time setup pails in comparison to the alternatives.