There is a special connection between the wizard and his familiar.  Yes, the familiar helps with the magic and makes the wizard powerful.  However, in return, the wizard gets something even more valuable, in return.  Friendship.  The life of a wizard is lonely and he has very few "friends" that he can trust.  He creates a bond with his familiar (if he is good) and that bond enhances both of their lives.  The familiar becomes more like the wizard and vice versa.