Why is it always required to use position when the parent class is relative

When we use position, the outside frame is generally used relative, the elements inside use absolute, here are two points to note:

1. If we do not give the parent class A position attribute, then the child element will take the body as a reference, and then the thing will automatically "run away", so the position of the parent class is necessary.

2. If the parent class uses a position that is also absolute, the parent class will also be separated from the document flow, so that the parent element does not occupy the corresponding space causing the element to crash, that is, the individual elements are placed in a disordered position. And because the parent element is absolute, the corresponding object becomes body, resulting in either you can only finely calculate the position of the element, and then "out of thin air" to put up.

Why is it always required to use position when the parent class is relative