Not everyone has a fancy client-side MUA that gives them trivial access to mbox files. E.g., a typical webmail service will make exporting mboxes into a whole process at best. (And on the sending side, have fun with the lack of git send-email integration. I've spent far more time than I'd like manually patching up References and In-Reply-To headers.)
Of course, the classic response is "get a better MUA you luser", but that just adds even more steps for people who use webmail services for 99.9% of their email needs.
Of course, the classic response is "get a better MUA you luser", but that just adds even more steps for people who use webmail services for 99.9% of their email needs.