I have all my stock plants (used for taking cuttings to propagate and sell) in my house. I keep the cuttings and starter plants in the Greenhouse. The plants in the house, no matter what kind you have when you keep them inside they will gather "dust" the same as the furniture. I also have caged exotic birds in my house and they create some dust. I have always at least once or twice a year, on the big ones wipe the leaves with plain water one leaf at a time and on the smaller ones, take them to the sink, tub or wherever and spray their leaves and clean them off. I was always told it helps them breathe....then again, I am old and we do things differently than many younger people.
Giving them a bath also helps remove spider webs, mealy bug or other insects you may not notice just looking at them. I do it this time of year because I have the time. Also, many of them will go outside for the summer and they will get washed by the rain during the summer months, but when I bring them in, they get a bath to be sure there are no unwanted critters coming in with them. I have quite a bit invested in these plants and I need to keep them healthy for cutting when it is time.
I always think I feel so good after a good shower I want my plants to feel good too.
You are no doubt thinking they should come and take me away in a white jacket, but that's ok. My plants are clean and healthy because of this. I hope this helps explain why I bathe my plants twice a year.,