The export function takes all the Flight information for a route and exports it to a file. You can then import that file to another route in the same mission or even a completely different mission.
For instance, say you have a set of 4 flights of B-17s in a perfect box formation. When you export it you can then use that same box formation in other mission files. All you have to do is set the spawn point without having to re-enter offsets, loadouts, fuel levels, spawn formations, or flight descriptions.