The marquee tool in Photoshop has three modes: normal, fixed ratio and fixed.
Usually you will use normal marquee to select whatever you want, but sometimes you just don't need that kind of freedom, you want an exact fixed rectangle, instead adjusting it carefully with your mouse pixel by pixel, its more efficient to define the precise size beforehand.
Here is where the fixed marquee tool comes in, it allows you set the size of the marquee tool to whatever you want, so you easily create a fixed size selection without manipulate the mouse.
Suppose you created an icon, which must fit into predefined CSS style which has fixed width and height.
