Most of us go to great lengths to keep our gear nice and clean. The last thing you want is for your lens to affect your images simply because of how downright filthy it is, unless that's exactly what you want. This video shows you how occasionally letting your gear get dirty can be a good thing.
Dust on your sensor? Please, no. Water on your front element? I get fidgety just thinking about it. Most of us hate the idea of our gear getting dirty, and especially dirty enough to effect our image quality. Even so, Evan Ranft has another perspective. In this video, Evan talks about allowing some typically unwanted environmental circumstances to enhance photographs and make the viewer feel like they're more involved in the scene.
A photo can easily be successful if you take a straightforward approach and photograph your subject and only your subject. However, using something to enhance your subject like some raindrops on a lens or an object in the foreground that has a relationship to the subject can visually join a series of images together and can certainly help the viewer make visual connections while creating a story in the process.
To be fair, Evan clearly states he doesn't recommend keeping your gear dirty to hopefully create an interesting effect. Instead, he encourages being open to the environment and making use of objects and situations that can bring the viewer deeper into the scene. Still, that can sometimes involve allowing the environment to get up close and personal with your gear. Just don't get carried away.