Answer:
A wedge is used to take advantage of the fact that work done = force x distance.
Simply put if you need to force two points apart a wedge allows you to use less force by moving a greater distance.
If you knock a wedge into a piece of material by 10cms in order to widen the gap by 2cms it will take one fifth of the force to achieve this.
If you think of a screw as being a nail with a very long wedge wrapped around it, you can force this into a solid piece of wood with one hand simply by taking advantage of the mechanical advantage that this wedge provides.