What are the ghost's functions as a dramatic device in the plot of Hamlet?

The guards' talk of the ghost on their foggy, gloomy night watch at the castle and the ghost's appearance at the end of act 1 make for a dramatic and memorable opening to this play, along with vivid imagery of eyes bugging out of heads and hairs on heads unfurling and standing on end.
More importantly, the ghost's appearance is what sets the plot in motion. Up until this time, Hamlet has been depressed by what he believes was the accidental death of his father. Now, he confronts a ghost who claims he is Hamlet's deceased father. The ghost tells him he was murdered by Hamlet's uncle, Claudius. With this news, Hamlet is expected to avenge his father's death.
This revelation throws Hamlet into a turmoil for several reasons. First, he has to determine if the ghost is telling the truth or if it is an evil spirit tempting Hamlet to murder an innocent man. Second, being confronted with such evil as the deliberate murder of his father by his uncle causes Hamlet to distrust the corrupt Danish court and to contemplate suicide. Finally, Hamlet's desire that Claudius not die in a state of grace but unforgiven, like his own father, leads Hamlet to miss the opportunity to kill Claudius at his prayers. Instead, he ends up killing Polonius by mistake, bringing Laertes and his lust for vengeance down on Hamlet. The ghost, in other words, is the catalyst for all the action that follows.