You might find it more intimate being completely naked with someone, like you're not exactly naked often so you might consider it more special like that.
But it depends on the person, what you feel happiest with / most excited with and your views on nudity and your / their body
So is penis, or the more family friendly willy. Half the world have one
Fully naked best, but sometimes not required for a quickie. Just make sure you take your socks off!!!