When paying by card flight attendants usually ask to see your ID, to verify that the card in fact belongs to you. However it seems weird to me that pretty much no other merchants ever ask to see your ID, even when conducting purchases for thousands of euros (e.g. at IKEA).
So why do airlines insist on verifying your ID when paying by card? I understand that there's (usually) no Internet to verify the card balance on the plane (as suggested by TonyK), but how would one's ID help them out in case the card is empty?