I also thought it was that way..but it does makes sense. The Notebook's fan fools the CPU of the notebook by sucking air from the bottom of the Notebook. Therefore, the cooler the air at the bottom of the notebook..the better it is. Either putting the notebook inclined or giving more space at the bottom will also do the trick. Laptop coolers will do it better.
Now, if the laptop cooler is blowing (instead of sucking) air into the bottom of the notebook, there is a tendency for dust to accumulate at the bottom and clog the inlet of the notebooks fan.
