So, what's Dublin got to offer that Manchester doesn't on a route like this?
Pre Clearance.
Due to the topography of Addis, plane has to stop en-route to LAX. May as well get the passengers cleared at DUB while the plane gets juiced up.
That way, Ethiopian has the right to use domestic gates at the lax end.
Indeed. Same principle as BA's LCY-SNN-JFK route: it has to stop anyway (refuelling and longer rwy) so may as well use the pre-clearance facility.
Makes sense, and, surprisingly, only 9 mi. longer than a direct great circle route (according to great circle mapper).
Great Circle Mapper