The big reason is to provide an (automated) vertical path / guidance that the FMS can follow, even with older DME/DME based equipment.
Not an AirBus guy, so some info please. The statement above would suggest an FMS guided approach. Does that need autopilot in CMD or can it feed into the FD? Is this FMS approach used to give guidance until you intercept a visual centre-line & visual glide path?
What we have not yet heard, and perhaps only the crew will know, is if they were in manual control or autopilot during the initial approach, and at what point they went manual. It is suggested that the FMS gives vertical guidance, but we are shown bright shiny PAPI's and it is a visual approach, certainly from 4nm or so. That would suggest PF would be scanning out of the window and inside to PFD in varying ratios descending below 1500'. I would have thought that from 1200' or so the PAPI would have been the primary vertical guidance aid, and I would have expected PF to be in manual control from that time, at least. Thus I am curious how the PAPI did not alert them to the active runway. The taxiway is a couple of hundred metres laterally displaced from them; and between the only PAPI visible and the taxiway is an equally bright & shiny only 1 set of approach lights. If they were following the PAPI in vertical how come they were following a flight path that would have landed them well into the taxiway, and not, seemingly, abeam the TDZ of the runway? I can't believe AirBus guys use the FMS FD guidance for vertical path below 500'.
So what might they have been using for vertical guidance below 1000' if not the PAPI?