Fossil impressions typically preserve the INNER parts of a creature, because the pressure destroys most everything but the bones. For example, if a bird dies and forms a mold or cast, you get its feather imprints, its shape, etc. However, if the bird becomes an impression, you might see some feather impressions, but you lose its shape entirely, because it is crushed. In addition, you typically see the skeleton, which is the inner part of the bird.