This is a serious question, why don't you just use a sensitivity that is comfortable for you? Clearly if you need a button to change sensitivities in the middle of a game you are using a terrible sensitivity for your needs.
Well, when "running around" in a fps, it would be nice to be able to hit one button one time to bring the dpi waaay down. Having this "sniper button" feature would allow playing at a dpi setting you're comfortable with but then allow, with one click, incredicble precision when pulling into scope mode on the weapon of choice wihtout having to multi-tap through 3, 4 or 5 five dpi settings.
For instance, when in a tank, I prefer having a higher dpi setting so as to turn the turret as fast as the game engine allows without having to move the mouse across the desk, the floor and out the window (for those times where say someone is firing at your from behind and you need to turn as fast as possible to return fire - while of course moving the tank out of harms way). But while in this same scenario, be able, with one click, to precisely hit a target relatively far down range.
I've had plenty of times where I am trying to use a tank's .50 caliber to take out a soft target at a bit of a distance but can't quite hone it without going too far to the left or right of the target. It then turns into a fumbling through the dpi settings (5 of them) until I get to the correct one. And usually by then the target has left the area.
As fayd said, it is nice to have on the fly dpi settings, so why not take it one step further?