Well, some people change their water infrequently, and for them it may be more advantageous to skim wet. Skimming wet will strip the water more, and is not necessarily good. Dry skimming removes more of the waste and less beneficial bacteria in comparison to wet skimming. Wet skimming is good if you have a fish die, or an anenome that may be a touch angry. I prefer dry skim, but it just never works out that way, so it's a tad more wet than I prefer.