Why is there the difference between the primogeniture and the cadet branch in a royal family?

"Primogeniture" means "first born." In aristocracy, it is the inheritance of titles and lands by the firstborn son. All lands and titles are conferred to him and to him solely. This best ensures the survival of the family's power and position by keeping the land and titles consolidated in a single entity exactly as it was conferred to that primogenitor's primogenitor predecessor.

A "cadet branch," on the other hand, is formed when a non-first born son, a later child, is granted lands and their own title. This does not always happen. It's actually somewhat rare. That's because the point of primogeniture is to maintain the potency and concentration of the family's wealth and so its standing within the aristocracy instead of having the wealth and lands constantly being divided out to more and more people until it disappears into the wind, completely eroded away. So, you get cadet branches when a family has greatly increased its holdings and titles and has them to spare, has them in such spades that what the primogenitor inheritance will be at least or more than what his predecessor inherited.

Succession-- the primogeniture branch is the branch that inherits the throne---the heir is the first born--- from the Latin primo for first, genitūra ,a birth.

The eldest child of the monarch is the heir apparent. If the eldest child is married and has children, the offspring follow in line of succession in order of birth. The cadet branch consists of siblings, cousins, other relatives who are lower in line of succession, and not likely to inherit the throne.

Because that is how they decide to whom the throne passes. Normally it goes to the eldest, usually male, child. The other children can inherit in some families and they are the cadet line. If there is no heir in the senior line the throne may pass to one of the other children in order of seniority.

For the obvious reason that the throne passes down through the line of the first-born in a monarchy that observes primogeniture -- and most do. A cadet, or collateral, line belongs to younger siblings who are not directly in line for the throne, though someone in a cadet line could inherit if a first-born had no children. It's happened often enough.

Is there? bet there aren't 5 people in the whole world that care.

As king, Charles will abolish the York and Wessex cadet branches of the current royal family which are led by his niece Beatrice and nephew Viscount Severn. He has no concern for the non-royal female branch offshoot of his sister Anne, led by the non-royal Canadians Peter and Amber Phillips.