Q: "Rats of Rath" have a special ability that allows the owner to tap 1 Black in order to "Destroy target artifact, creature, or land you control." Why would anyone want to destroy a permanent that they control? This does not sound like a very useful ability.

A: Sometimes its advantageous to put one of your own cards in the graveyard. For example, if you are just about a spell called "Living Death," which puts all creatures in play in the graveyard and all creatures in the graveyard in play. While conditional, there are numerous times a good player can make use of this ability.