Your solar panels will work better when they are clean. When they produce more energy you will receive more money via your Feed in Tariff. If you have a commercial solar farm you may have commercial agreements based on other criteria, but it's unlikely to be in your interest for the panels to be producing less energy than their predicted output.
Solar panels are relatively new and technology changes. Dirt, bird droppings, dust and grime is nothing new and we are experts at removing it from hard services. Here is a link to a solar panel advisory company that gives you more details about the efficacy of clean / vs dirty panels.