A big meal before significantly raises the odds of one or more participants having some kind of embarassing bodily function (burp, fart, etc.)
A meal, calories, by definition provides energy so if you've been out all night and never got a chance to eat or you are having marathon all day in bed session, clearly a little snack is a positive thing.
I will say, in the civie world, cooking for a woman is much more likely to get her pants off than buying her dinner at the priciest restaurant in town....