Nudity can become very commonplace when involved in the fetish community, but there are still some psychological components to it. It may sound over-the-top, but don't forget why we have clothing: for protection. No matter how comfortable you become with your own body, there's still an element to clothing that extends beyond modesty. Depending on the sub, forced nudity may or may not be a significant contributing factor but it can, at the very least, enhance a feeling of vulnerability. Personally, having the Dom fully clothed amplifies the effect for me, especially if I'm having to do something degrading like servicing his feet.