When a car's air filter becomes dirty or degraded, it is the quality and quantity of air supplied to the engine that suffers. By obstructing the combustion chambers, contaminated air interferes with the proper combustion of fuel. The engine then works harder, which leads to overconsumption of gasoline. Not to mention that its performance is weakening. You can imagine that all of this damages an engine, the same way our lungs get clogged up breathing dirty air. Changing the air filter is therefore essential.
A poor air filter damages the internal engine ducts, which in turn affects the acceleration of your car, as well as darkening and thickening the smoke from the tailpipe. You could end up with a lot of problems on the arms : overconsumption of fuel, polluting fumes and even engine failure.