I'd make a reasonable guess that it enables much better Javascript clients, either via Electron or the Web Browser.
You don't need major providers to support it, they support SMTP and that's how messages are relayed. JMAP is just so you: the client, can fetch your mail from wherever you host your mail.
You don't need major providers to support it, they support SMTP and that's how messages are relayed. JMAP is just so you: the client, can fetch your mail from wherever you host your mail.