In vanilla minecraft, the wither and the enderdragon can destroy the blocks in their path. However, they can't destroy a few blocks (bedrock, command block, extended piston...) and they just pass through them like if they were ghosts.
It would be cool to be able to give the ability to pass through blocks to a custom mob.
It will make boss battles harder, allow for the creation of mini-bosses that don't destroy blocks and special ghost-like monsters.
It will also pair well with flying mobs (when they will be added).