Mythology often speaks of a witch changing her shape into a cat and although this is obviously symbolic as opposed to actual, it says a lot about how powerful witches in times gone by were perceived to be. The cat has become particularly associated with witches as many of the old women accused of witchcraft owned a cat.
The familiar is said to be both the servant and the protector of the witch and the cat is an additional pair of eyes for the witch to search out dangers
Although modern witchcraft still embraces the concept of the familiar, I must stress that a cat is not a necessary part of a coven. Not all witches have a familiar and those that do no not necessarily use a cat. I own four cats, none of whom has every demonstrated any ability to assist me with my magic. To be honest, I’d consider my tarantulas to be better familiars than my cats.
Image http://www.sxc.hu/browse.phtml?f=view&id=1417153 (merlin3)