Just as mscupid said, smoke rises to the top. Inhaling the smoke will fill your lungs and render you unconscious.

 Because heat and smoke will rise making it easier to breath on the floor so it is easier to get out of the burning room!

 Because smoke rises just like heat rises, therefore there won't be (or there'll be LESS) smoke at the bottom making it safer for you to crawl out to safety with few injuries.

 As the 2 answers here before me, the smoke is rising to the ceiling, and the when it dos not have place to go higher, it starts to build down towards the floor, so in a typical house fire, you will have about 5 to 7 minutes to get out of this houseroom, the reason for anyone to try not to breathe the smoke is that there are Cyanide's particles in the smoke, this stuff can kill a human being in jest a few minutes, so you need to open all of the outer openings of the house, (windows, doors, etc...)