Rain alone will not clean your panels, just as rain alone will not clean your car. Professional cleaning of your solar panels ensures that they will function at their most optimum level of performance.
Solar panels are designed to capture sunlight and then convert it to usable electricity, so anything that limits the ability of the panels to capture this sunlight will decrease the amount of generated electricity. Things such as rain, dust, urban pollution, and bird droppings can cloud these surfaces and affect their efficiency. Regularly cleaning not only increases their efficiency but also provides savings for the system owner. It is estimated that regular maintenance can provide up to 20% more efficiency.