The flight reservation is a formality one can not simpler and not very useful for the embassies, It can only be issued by a travel agency. Rather than buying the tickets the agency takes care of reserving them, A very important nuance in this case since the booking does not require any payment of the ticket.
Through its reservation system, the agency will propose to you to book a round-trip airfare on the dates of your choice, this is only a booking, which means that you will not have to take these flights there or even to cancel your reservation. 

To put it simply it is a simple formality to carry out for the embassy. Travel agencies are accustomed and know it. And for hotel reservations, it's exactly the same principle.