Two human personalities can form a relationship, and, through communication and communion, that relationship can be enhanced. Likewise, a person can form and develop a close relationship with an evil spirit, whether for knowledge or gain. When a person forms a relationship with an evil spirit, he then has a familiar spirit.
Familiar spirits are counterfeits of the Holy Spirit's work. Anyone who has fellowship with God, and who is led by the Holy Spirit, does not need a familiar spirit, for God provides him with all that he needs.
As with anything occult, involvement with familiar spirits creates spiritual defilement: "Regard not them that have familiar spirits...to be defiled by them: I am the Lord your God." Lev. 19:31