Thanks for your answer Nicolas.
I just want to add that the power reserve display has an interest here: to gain space to put a slimmer but longer mainspring, Lange removed the Maltese Cross on top of the barrel which purpose was to insure the optimal amplitude. The Maltese Cross was removed and so a power reserve display becomes useful to be sure to wind the watch every day to get, the best behaviour.
You will argue that a Datograph owner will forever wind it very morning due to the nice handwinding feeling. But with a larger power reserve, some may not feel the need to do it every day. Not true Lange fans in that case !
Fx