Q:  What purpose does Satan serve?
A:Satan serves a contrasting figure. God is omnibenevolent, Satan is wicked. His purpose is to tempt human beings of free will to commit evil acts to test our faith and turn us away from God, who he despises.

Q:Do you believe in Satan? If yes, how would you describe Satan? If no, why do you think people believe in Satan?
A: I'm sure there is a Satan, but thats about all I'm sure in that regard. His relationship with God is unclear to me. His appearence, where he is, what hell really looks like....is pretty irrelevent in my opinion.

Q: What is the relationship between Satan and evil?
A: Satan represents evil,  he promotes all evil acts because they draw people farther away from God.

Q:  Do we need Satan in order to believe in God?
A: No, not necessarily. However, it would either mean that a) God isn't omnibenevolent, or b) there really is no such thing as evil, or good... or c) everything that happens, happens for an overalll, higher good.
It gets very far in depth philosophically.