Flag Dropping is a common tactic performed in the Capture the Flag gametype when the flag carrier has no access to a vehicle; since carrying the flag reduces the player's speed, repeatedly dropping it ahead and grabbing it is a precious time-saver, especially when time is running out.
Flag Dropping is also used as a form of match-time celebration. When someone steals a flag, they may drop and steal it constantly while on their way back to their base. This leads to the narrator repeating "flag dropped" and "flag stolen", to the annoyance of the other team.
The tactic was almost completely killed in Halo: Reach, due to a new gameplay mechanic that only lets players build up normal running speed again after one second of dropping an objective item. It was officially killed in Halo 4 when the developers made it impossible to drop the flag.