1. In greenhouses, gardeners often spray plants with pesticides, thus dipping down the condensation stuff. Carefully check the plant after buying: you can notice white or gray spots on the leaves of your plants.
2. To get the dirt and dust off! Sometimes, it’s not a big deal until it’s building up heavily and blocking the way to get oxygen. As plants breathe through their leaves, you always have to check whether they are as clean as possible or not.
3. Another reason is to get the residue from pests off the plants. If your plant has suffered from diseases (feds, mealybug or something like that), it's gonna leave behind a sticky residue and there might also be some eggs (oops). That's going to be a good reason to clean the plant and get rid of all those things.
4. ..for a good look. To look at your plant and see it clean just gives you another feeling.