What are the advantages of using a keyboard and mouse for first person shooters when gaming on a PC?

Nearly all professional gamers prefer to use a combination of keyboard and mouse as input controls when gaming in first person shooters (FPS for short).  The major advantage comes from using a high optical resolution mouse to control the player’s viewpoint.  In the FPS world, this translates to targeting an opponent.  The mouse allows for extremely fine grained movement of the viewing reticle to hone in on specific areas such as the target’s head.  Hitting the head often immediately results in death for the target or significant loss of health.  Additionally, the operator has access to other keys on the keyboard for quick access to a wider range of functions which differ based on the game.  Although a two-handed controller features a much easier learning curve, it cannot compete with the precision offered by a keyboard and mouse combination