There are several types of familiars, or magistelli, to give them their more correct title. Basically, there are three types of magistellus.
The third type, and to the individual witch (or wizard), most important type of magistellus is that of the magical servant. This may be a demonic entity who has been "bound" to some magical instrument such as a show stone or mirror, permanently or temporaily. It may also be an elemental creature formed by the combination of your witch (or wizard) power and some natural phenomenon. This type of magistellus becomes the protective spirit or Watcher, a magical guardian of the home. They take time to formulate, but really can prove to be of tremendous value. For unlike the general Earth spells which are effective to guard your home from general bad vibrations, to guard against out-and-out magical attack, a magistellus is ideal; it possesses a definite will of its own, the entire aim of which is to protect the house and those that dwell in it from all offensive sorcery. As such, it is really a magical type of vigilant robot, programmed solely to watch over the safety of the home. A sorcerer's watchdog, in fact.
Interestingly, in Jewish tradition, the term golem (a creature created "artifically" by magical means) is also used to describe newbies to the tradition and Torah study. In other words, ba'alei teshuvah and converts are often used as golems, magistelli and familiars are used. They are "simple" people, in terms of Torah scholarship and are eagerly looking for acceptance into a community. Consequently, they can easily be used as watchdogs and to "bide the issue" as discussed in a previous entry regarding lost curses.
In my own way of thinking, a wizard or witch ought to be willing to take full responsibility for his or her magic. If he or she isn't willing to do this, then something is terribly wrong with the kavanah and/or magic of the wizard or witch, and clearly he or she suspects that the magic may have a cost that he or she is unwilling to pay.