Good heavens! What kind of ill-raised person shows up to a party without something to present to the host or hostess??? I agree that it is polite to ask what the host or hostess needs, and equally polite to say, "Oh, you don't have to bring anything." It's like when someone says, "I really appreciate this," and you say, "Think nothing of it," no matter how enormous the favor was. The idea is to present ourselves as civilized human beings with the ability to be hospitable no matter what.
People should know that it is awfully gauche to show up to a party or dinner without bringing a bottle of wine, some flowers, some type of baked good, etc. The main courses and accompanying dishes would be provided by the host or hostess, of course, and an invited guest shows his or her appreciation for hospitality with a little something extra.