Not to get too technical but it helps with fare calculations sometimes. It also may allow them to approve certain booking classes separately from the LH availabilty. In complex itineraries you may need to book a particular carrier code to allow the fare to price and this allows it to happen. Then there are different types of code shares. I think you can just book the AC code share on TATL LH flights without the need to have any other AC segments. However, on the intra-european flights - the code share cannot be booked without the linked TATL flight. That being said - on the intra-european flights - the AC fares will price the same regardless of which carrier code you book within Europe (OS/LH/LX). In one example (for an LX ticket) we needed to get to BOG out of ZRH and the fare required LX code - we got the LH flight FRA-BOG on LX code - without that code being there - it would not have priced.