Why is there so much precasting?

EMAILNOTICES>noDon't deny it doesn't happen. How many times have I seen more than three times the necessary people audition and many people who deserve the part to see one of the director's relatives (wife/husband, daughter/son, brother/sister and so on) get the part, despite the fact their audition was actually inferior. And it's not because they the right "look" either. Why bother auditioning the part if the casting is a foregone conclusion? Is it just for show?