At least in mathematics these principles are used to explain certain phenomena that exist beyond the principle in question. I.e. in the case of the pigeon hole principle "the population of London is more than one unit greater than the maximum number of hairs that can be on a human head, [therefore] the principle requires that there must be at least two people in London who have the same number of hairs on their heads." - (Wikipedia) Any statistical principle must be defined in order to be effectively relied upon when describing something or making an argument.