The HVAC System is one of, if not the largest consumer of energy in a building. Having clean coils allows them to operate more efficiently which minimizes cost. A cooling coil should be able to lower the temperature of air across it by about 20 degrees. Not only will a dirty coil not be able to accomplish that but, it will also put back-pressure on the blower motor causing it to work harder to move the air. For example, if the static pressure across an HVAC coil increases by 25%, the blower motor uses ~56% more energy.
A dirty and wet coil can also become a breeding ground for molds and bacteria.