Contrary to common opinion, paint is not completely opaque. If you were to paint a window and look through it you would be able to see some light passing through. the more coats, the less light you’ll see. When applied on a wall, light has a tendency to pass through paint and reflect the color from underneath back out – what most people think of as ‘bleeding through’. The more dramatic the color change, the more likely you’ll need a thicker layer to block out the original color.
Poor tools or improper application can also cause a need for extra coats.