The hard, straight, heavy, dense coat  is, of course, the very coat that the Lhasa needs for protection and insulation. A certain amount of felting of the undercoat may be advantageous to the animal as a method of wind proofing. The correct straight, hard, double coat would still leave most of the topcoat open. Shed undercoat in this type of coat tends to move to the ends of the guard hairs and be cast out naturally. Incorrect silky or woolly coats would offer little protection, and dense matting close to the skin can cause sores due to traction on the skin. A trait of many Lhasas, the bane of the exhibitors existence, the tendency to groom themselves by trimming and combing their own hair, probably has an important survival function for a long coated dog which receives little grooming from its human companions.

In warmer climates, Apsos tend not to develop as much undercoat as they do when kenneled outdoors in colder climates.  Their human caretakers also often deliberately strip out substantial amounts of this undercoat for ease in grooming.

The correct Lhasa coat is the most obvious casualty of the showring. Handlers prefer a softer more voluminous coat, which can be sculpted to conceal or alter the outline of the dog. In Tibet, these coats would constitute a serious unsoundness, which is probably why travelers to the region report seeing so few animals there with this type of coat.

There is some misunderstanding of the term "of good length". The standard is not asking for a perfectly trimmed coat dragging on the ground, or even floor length. The standard is requesting evidence that this is a longhaired dog, which is longhaired all over and not in a pattern like an Afghan. To penalize a dog for its youth or lifestyle, or because of the inexperience of its groomer, will never improve the breed. In fact,  the practice of rewarding overgrooming is detrimental to the health and well-being of the animals, some of whom literally spend their lives sitting on tables or in crates so they will not break a hair. But the Apso is a watch dog and companion, not an ornament. A dog exhibiting a good quality of coat, with sufficient length to demonstrate its overall pattern, even though it has some chewed or broken spots from rough play and outdoor exercise, is much to be preferred to an impeccably presented animal with an immense coat of improper type.

The correct coat will have shine due to the large diameter of the individual hairs and the smoothness of the cuticle of the hair shaft.  When rubbed between the fingers it feels "grainy" - you can feel the individual hair fibers.  When the coat is picked up and dropped, it falls as if it has real weight - which it should have.  This is the meaning of "heavy" in the standard.

Here is a life-sized photo of a correct Apso coat.  This is the coat of an 8 month old puppy, already developing adult texture.

Here is another view of correct coat texture.  This coat belongs to a six year old male.  The magnification in this photo is about 4x the previous photo.