https://www.schengenvisaitinerary.com/blog/schengen-visa-itinerary/importance-of-travel-itineraries-and-flight-itinerari

Flight Itinerary document when making a visa application is mandatory at Schengen visa embassies and consulates. It’s wont to confirm that the applicant is really meaning to visit the places on the dates stated. The document is additionally wont to show that the applicant is really meaning to make the journey back. Provide proof of intention to urge back on the dates as stated on the appliance form.

For example, if you've got claimed in your visa form that you simply are meaning to come on a particular date, but you've got provided a one-way travel itinerary, then your visa application is probably going to be declined due to the inconsistency of intentions.
Full Info Link: Importance of Travel Itinerary