To produce energy at optimum efficiency.
Anything on the surface of the solar PV panels decreases the amount of sunlight (photons) striking the solar cells and will result in decreased energy production. Foreign particulate matter such as dust, weeds, pollen, bird droppings, tree leaves & sap, spider webs, moss or fungus, ash from fires and other contaminates. All of these decrease energy output.
If you live in an area with strong winds, your chances of accumulating dirt will be much higher. Dirt can also be transported through rain as it falls through the atmosphere picking up impurities in the air.
Dirt on black panels is almost impossible to detect. You must have regularly scheduled cleanings, a minimum of two times per year, according to reputable manufacturers.
Try taking a clean white tissue and wiping your car windshield after a rainy day. The dirt left behind is evident. Leaving the dirt to accumulate, it will slowly but surely combine with moisture or other oily substances from the surroundings to form a denser layer of dirt. Regular cleanings will prevent the buildup of dirt. Sap and bird droppings are very destructive. Left to bake in the sun, permanent etching or deep scratching will cause critical damage.