I don't quite get the concept of local mail in the 21st century. Long gone are the times where we had exactly one computer. What good does the e-mail do me if

I don't quite get the concept of local mail in the 21st century. Long gone are the times where we had exactly one computer. What good does the e-mail do me if it is local on (one of) my desktop(s), when I need it on my laptop/phone/pad? What's wrong with IMAP?