Tongue weight lowers the load applied to the front wheels, hence compromises steering ability and grip, particularly in situations of lower than usual adhesion, eg. rain,loose gravel. Usually, the vehicle has a max load limit that can be applied to the towbar, therefore the tongue. The trailer coupling has a max load limit as well. The general opinion is that the vehicle and the trailer should be weight neutral, that is, minimum weight on the tongue. Vehicle pulls itself up, trailer pulls itself up, guided by the vehicle, not stopped by the vehicle, except in some lower trailer weight situations that do not have brakes, eg. the garden trailer.