Be able to place advertisements on tv, radio, newspapers etc. which will boost popularity to the airline or specific flights themselves.
Please read here
http://community.airlinesim.aero/topic/6023-an-idea-for-marketing/?hl=advertisements
or here
http://community.airlinesim.aero/topic/3136-marketing/?hl=advertisements#entry23878
or here
http://community.airlinesim.aero/topic/1319-marketing-ideas/?hl=advertisements
Believe it or not but in the old GUI there was even an page for this though there was never any advertisement.