The principle of specificity applies not only to training, but just about any domain, field, or assessment.
"Teaching to the test" used to be considered a bad thing thing in education. People (correctly) thought that there was no value in, for example, memorizing a list of arbitrary facts simply to regurgitate them for a multiple choice test or quiz. If the test has no 'real-life' application whatsoever, then there will be no value in teaching to it. HOWEVER, if the measuring stick is a valid one, that demonstrates, develops, and tests student knowledge, skills, and motivation, then teaching to the it may have great value.
This does NOT mean, however, that training or education should consist of endless assessments and tests. Without proper preparation, reflection, and development of skills, even the most authentic and valid test will be worthless - for example, although a squat one-rep max is a good measure of overall body strength, it would be silly to have a trainee do it frequently when something less intense may serve the purpose just as well. In fact, over-testing, especially when the test is particularly demanding mentally and/or physically, is a quick road to burn-out and stagnation.
Segmentation and scaffolding allow the trainee to take on a challenging task by breaking it down into its requisite parts and providing the support necessary to provide training stimulus, feedback, support, and motivation to try, try again.