If you don't know you need it, then you don't need it!
Some older games, such as Half-Life 1, Counter-Strike 1.x, Quake, Quake 2, Unreal and others, while they are active and running, call a Windows function intending to disable variable mouse acceleration by forcing ALL movement to be accelerated by the same amount (doubled).
On Windows 2000 and earlier, that removed all variable acceleration.
Pointing and aiming in those games was OK, because the mouse response was then linear (all movement was accelerated by the same amount; it was doubled).
In XP, Vista, Windows 7 and Windows 8.x, Microsoft changed how mouse pointer acceleration worked.
Now when those games call the function (asking that all movement be accelerated), Windows enables the mouse 'Enhance pointer precision' feature, which adds mouse acceleration using a varying curve to control the mouse response. (It enables it even if you have it turned off in the Control Panel Mouse settings.)
With 'Enhance pointer precision' enabled, slower mouse movements make the pointer go extra slow and faster mouse movements make the pointer go extra fast. It is not linear and not straightline.
This is annoying, because where you are aiming at depends on how far you move your mouse, and also on how fast you moved the mouse to aim.