Primitive Objects come unwrapped by default, you build up upon it and make a mess out of the original UV’s. All the while you are modeling and changing your mesh and then you unwrap and get the shape in the UV’s your mesh now has.
UV’s are fairly simple. It’s just as a cornflakes box when you open it up to throw it in the trash. You don’t want the 3D version of the box in the trash, it takes too much space, so you rip the box by the seams to make it flat and 2D. In Blender you are trying to intuitively think where the seams should be, because there are no seams. In the cornflakes box it comes with the seams already, in Blender you have to decide where a cut should be for an object to be lay flat if ripped like the cornflakes box.
