This is not to be confused with a tail-gate party, which generally centers around some sports-related enterprise and also features grilled meat products. This is a down in the dirt drink-fest, which is generally held somewhere where cops can’t find you (even if you are, yourself, a police officer).
There is a firmly held belief that this sort of thing makes the beer taste better or at least more fun to drink. Plus driving home later becomes more like a video game. One that features drunkards attempting to drive home in a truck.