Why shear?

Shrek, a sheep that hid in a cave for six years without being shorn;

The only purpose of shearing is not to extract wool as a raw matter, and I would even say that for most breeds kept by man, that is not even the main goal. Here in Portugal, where most sheep aren’t bred specifically for wool, and where most of the wool is no longer so valuable as it used to be, the animals are shorn anyway.
So, why are sheep shorn even when the wool has little comercial value and is actually more of an expense to remove than a benefit for the breeder?

Most sheep breeds are domesticated animals. This means that they were developed by man throughout centuries to correspond to one or several production needs - wool, meat or milk. Unlike the wild breeds, whose hair falls out naturally during the warmer weather, the wool on a domesticated sheep grows all year long. It simply does not stop growing.
A sheep that isn’t shorn, and that keeps its wool for summer, will probably overheat and loose the ability of regulating their body temperature during the warmer weather. Adding to that, there’s the increase in weight that will be discomfortable for the animal and cause mobility issues.
Hygiene wise, during a whole year of growth, the natural dirt and a certain amount of vegetable matter, that dependes on the conditions where the animal is kept, will accumulate on the fleece. Because of this, the wool will get felted and get dags, which is also very uncomfortable. The rear end and genital area will also accumulate urine and excrements, that can cause burns and diseases on the animal’s skin, as well attract insects and worms that hide in the fleece and skin’s wrinkles.

So,  why do sheep absolutely need to be shorn? Because it is an essential step to guarantee their welfare.