Wet rub?

If you are talking about a rub with oil in it, it can make a small difference in lean cuts, as it adds a solvent to help extract the oils in the dry ingredients and make the dry ingredients stick to the meat. I feel rubbing the meat down with olive oil before rubbing accomplishes the same thing. The fat in other cuts seems to accomplish the same thing without using the oil.
I can get a more even covering of rub using it dry.