It build up more and more heat until it burns out the chips, caps, etc. kills the CPU because it has no air flow to cool down the CPU. Dust will stop the air flow getting to the required parts that need cooling.Would you run your car without the water coolant in it? No you would not, why because it will overheat damaging the engine. The some principle hear, think of the inside of your computer as a car engine it needs to be cooled. It will run, but running it hotter then it should be, will over time, damage the hardware inside your computer this happens, You would have to replacing the CPU, Motherboard or other parts could mount up to a lot of money.