What do sheepdogs actually DO?
A good sheepdog should be a valuable assistant to the shepherd or sheep farmer. It will learn where the sheep (or other stock) are likely to be in each field, and it will gather them together calmly, and not leave any behind. A really good sheepdog will even recognise if one or more animals is in difficulty, and attempt to draw the handler's attention to it.
Years ago, Carew (one of the best dogs we had) and I were gathering a large flock of ewes and lambs for our landlord but she was bringing the sheep unusually slowly. From a distance I couldn't make out why but as they got closer, I saw that one very small lamb was lame. Carew was walking alongside it, bringing the entire flock at a pace the lamb could manage.
Sometimes when Carew was going round the edge of a field to gather the sheep, she would stop and look at the hedge. I quickly learned that this meant there was a sheep in difficulty. Usually it would have its head stuck through the wire netting or something similar.
It'll seem obvious to many, but a sheepdog should move around sheep without disturbing them unduly. It should have the "power" to bring or drive them wherever you want it to without unduly stressing or damaging them in any way. Sheepdog Power is a thing we'll discuss later.
In the UK and many other countries of the world, a good sheepdog is expected to leave the shepherd or farmer's side and go out over terrain inaccessible by motor vehicles, to gather the sheep in a wide outrun. Without leaving any behind it will then bring them quietly and efficiently to the handler.
There are (of course) many other tasks the sheepdog performs, such as driving, penning, holding and pushing sheep through a "race" but the primary function is usually the outrun and bringing the sheep to the handler.