Besides the obvious reason – dirt, keeping your lenses clean will ensure that you get the best and highest quality results from using your gear. People just do not know how to clean their camera lens properly and after spending so much money on the camera gear, they seem to be too scared to clean it just in case they damage it. You should always carry a cleaning kit with you. We’ve done many comparisons over the years between a clean lens and a dirty lens and as expected, the first one indeed looked cloudy, while the second one was clear and sharp. This is one example of how dust, dirt and oil can affect your images.
Another important reason to clean your camera lens is to keep your images free of particles that might show up in background highlights and other parts of the image. Dust on the rear element of your lens will show up in your images, especially if you have large specks of dust there.