Why is overlapping of polygons terrible in graphics?

Question

3ds Max

Why is overlapping of polygons terrible in graphics? And how to avoid this when aligning objects on one line

Answer 1

But on the second screen you can see what it threatens.
If this happens outside the visible part of the scene - everything is ok.
If it is in the visible part, you will need to remove the "bottom" from the cylinder, or merge the cylinder with the plane, or place it at a small distance from the surface.