The purpose of a countdown to game start is presumably to give players a final, measured window of time to review the current configuration of the game they are about to begin as well as to alert players to the game's imminent commencement. This may be valuable in a time-intensive competitive game like StarCraft, in which wasting even a few seconds at the game's start can have a significant impact on a player's chances of success.
Some games allow the timer to be easily canceled once started ( Halo: Reach), while others do not ( StarCraft II).
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