God the Watcher
One common concern of theistic existentialism is the need to be watch in order to restrain ones freedom. Without this watching god, the sense of vertigo arises, in which one does not trust oneself not to go on a killing rampage. As an atheist, I often hear this argument directed towards me. This argument has a long history that goes before Existentialism, per se, developed, but it is a properly existentialist idea.
The Watcher allows a person to feel that no one can get away with evil. Some people feel a need for ultimate justice, a need everything in this world will make sense. This was one of the specific drives for Camus's concept of Philosophical suicide. The positing of a God is used to correct the lack of justice that is the condition of an indifferent universe. It is easy to conclude from the way Camus paints the subject's relation to the Absurd that philosophical suicide might indeed be inevitable given a desire for meaning, justice, and the lessening of the burden of freedom.
Intrusive Manichean Forces
Since gods are often used existentially to lessen the burden of freedom, especially given the burden of morality, supernatural struggles between good and evil are posited as well. This is where the Devil comes from. Temptations and other self-oriented facts of human life often get attributed to the Devil because of this distrust of the self.
In extreme forms of this, the self is seen as completely at the mercy of the direction of the struggle of good and evil that occurs in the self. In Angels in America, the character Joe when he describes the temptation of sinful homosexuality and the struggle with God as being analogous to Jacob wrestling with an Angel, wherein losing means you lose your soul, so you cannot win against the angel, but you must not lose. This struggle makes humans prisoners to supernatural forces, absolved of freedom yet strangely held responsible for the consequences. In the Disney version of Hunchback on Notre Dame, the villain absolves himself of responsibility for the temptation for having a gypsy in a similar struggle but between man and the Devil. His line "It is God's fault for making the Devil so much stronger than a man" is just one line in this song (see video below) of the villain relinquishing his freedom to external forces.