Pulleys will free up horsepower by increasing the leverage(smaller pulley) on the crank pulley.... and or larger pulleys on the accessories will do the same.

(another GREAT advantage is that a good crank pulley will be lighter than stock also)

warning----ANYTIME you use pulleys to increase horsepower you are lowering the speed at which your accessories pulleys spin, this could affect your enjoyment of daily driving...... for example, your alternator may have trouble keeping up under loads, stereo, highbeams ect.. you could also run into problems with your A/C not performing as well as stock.... stuff like that