Heat beds are used because they dramatically improve print quality by keeping the extruded plastic warm and thus preventing warping. Warping is a common condition caused by plastic on the edges of the part cooling down at an uneven rate when compared to the plastic inside of the part. The result is that corners warp up and deform your model. Heat beds work to prevent this warping effect by keeping your part warm during the whole printing process which keeps the material at or above heat-deflection temperature (the temperature at which it is malleable). Keeping the parts in the heat-deflection range ensures that the part remains flat on the print bed.