The underlying OS could already do faxes (by treating them as a special case of printers and popping open an OS-controlled dialog to accept phone number &c). Bu

The underlying OS could already do faxes (by treating them as a special case of printers and popping open an OS-controlled dialog to accept phone number &c). But Word bypassing all of that with its own fax drivers and protocols meant it could provide a full UX for the end-to-end of faxing (including, probably most importantly, scripting the fax send so you could merge from a phone number database)... At the added complexity of replicating an entire feature the OS already offered a slightly different way.