I was thought to keep the tanks because the fuel vapors are a lot more flammable than the fuel its. The more fuel the less vapors within the tank.
Re minimising condensation; I not too sure. Maybe if the aircraft is out side of a hot day with half full tanks the heat may cause the traces of water with the fuel to evaporate and the condense on the inside walls of the tank, this in turn may lead to water contamination of the fuel.
Please correct me if Im wrong.