Why do people brush their teeth before they eat?

I have always wondered why people brush their teeth before they eat and not after. Aren't your teeth going to get dirty when your eating? So why not brush them after you eat and not before.Some poeple say "When I get up in the morning i sometimes have a gross taste in my mouth and i brush my teeth." But don't you get that taste after you eat? I don't get it . Or is that the reason people brush their teeth in the morning before their eat? Because they have a gross taste in their mouth?