Why do witches have familiars?

Familiars are believed to be a physical embodiment of a spirit guide and/or an intermediary between worlds. I personally feel that the concept of familiars is a stereotype. I don't discount that it is possible for a spirit to take the form of an animal and act as a guide or intermediary for a witch but television and books have purported this notion to the point of it being stereotypical. A familiar could be a cat but could also just as easily be a lady bug. The majority of newbie witches feel the need to go out and get a cat name it salem and call it a familiar because they are uneducated as to what it a familiar actually is and are trying to fit into what they have been told a witch is supposed to be. I have been a witch my entire life and have yet to have a "familiar" in the sense that the world thinks of them as. I don't even particularly like cats. I'm more of a bird person.