For reasons of safety. If the birds were flying in a single line, the rearmost bird cannot see a hawk or other predator attacking from behind. In V formation, the birds towards the rear of one line can keep a watch on the birds in front, and at the rear of the other line, and call out a warning in the case of danger.
The safest position is at the front. But here the leading bird is acting as a pacemaker, and, just as in team athletics, this uses a lot of energy. (It's always easier to keep up than to lead). So after a time, the lead bird will drop back and give another one the task.