Watts ARE NOT HEAT!!!.
Ahem, Watts is the measurement of electrical power. The reason for those "ratings" is so you don't put too much of the old incandescent bulbs in which produce alot of heat.
another reason is so that you don't draw too much current for the cord it comes with. Say you have 2 70w bulbs using 110v power source. Using the formula P=IxE (where P is power, I is current [or amps] and E is voltage) we can determine that the total current = 1.3 amps, and if you have real small wires, the current can be too much, and starts a fire.