Fighter planes fly in formation to provide a tactical function. The V formation that I assume you refer to is called the Finger-Four, and was originally developed by the Luftwaffe in WW2 as superior to the Vic, or three plane formation, because it allows the squadron to split into two tactical groups each consisting of one leader and his wingman. The two ship element is of course the fundamental unit of air warfare where the leader engages the enemy while the wingman assists and protects. Two elements allow geater advantage where one element performs the role of "leader" while the other functions as "wingman". Flying close together in formation allows the group to visually cover all approaches (one plane scans each cardinal direction). Just like the infantry moves in squads and not as individuals, so do aircraft fly in formation. Larger groups are exceedingly rare in modern warfare but can rarely be used for carpet bombing to concentrate bomb damage.