Anyone who has ever turned on a laptop computer for more than ten minutes can see how hot they can get. Reaching operating temperatures of 100 degrees or more, this can cause electronics to malfunction or permanently damage them. A laptop cooler is an inexpensive way to help keep you machine around that much longer.
Desktop computers have a built in fan to keep them cool. Due to the size and portability of laptops, there is no good way to incorporate a fan into their construction, this is one solution that makes sense.