Upvote:3
From the looks of it, they probably want you to have an idea of where you're going, whether it's within the Schengen area or not, whether you need a particular visa, etc.
More than likely it's to prevent someone from spending a lot of money for a plane ticket that may not be used (if the visa is rejected).
Have a particular flight and/or date in mind, just in case. Good luck.
Upvote:3
It's definitely not obligatory as far as Schengen regulations are concerned, which is why they can't explicitly demand it and only recommend one.
Beyond that, you need to establish two things: that you have a valid purpose (that's where the itinerary comes in) and that you have the means to leave the Schengen area (that's why a reservation might be useful). The more evidence you are able to provide, the better.