Purposes of cleanliness are health, beauty, absence of bad smell, other comfort, avoidance of shame, and for avoiding spreading dirt and germs to oneself and other people, places or objects. In the case of windows the purpose is also transparency.
People usually wash themselves. Little children and sick and disabled people may be washed by someone else. For fun lovers etc. may wash each other. Often a shower is used, and/or a bathtub. More frequent is washing of just the hands, e.g. before and after preparing food and eating, after using the toilet, after handling something dirty, etc.