Originally Posted by zrh2yvr

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.