The flight itinerary is a document that shows a confirmed flight to and from the country you intend to visit. The office granting the visa has no wish for you to waste money on an actual flight until you know that your visa application has been successful, so this is just a reserved or dummy ticket to show your intention. Some consulates and embassies require you to show the actual ticket on the flight that you have booked when you pick up a successfully granted visa, but up until then just an itinerary with no confirmed booking is sufficient.