Dirty condenser coils make your system work much harder. This make sit use more electricity to operate and will lead to premature component failure. Primary causes are being located outside. Even worse when near cottonwood trees or dryer vents.
In this picture, you can see there is a gap above the filter even with a correctly sized filter installed. This allows dirt to collect on the blower wheel, furnace and coil greatly reducing system efficiency. Cause is a poor install.
This is a dirty secondary heat exchanger on a high efficient gas furnace. This dirt buildup takes away from the high efficiency and can lead to premature failure of the heat exchanger. primary causes are poor filtration and poor install.