Why is there a play, The Mousetrap, within the play, Hamlet?

There is a play, The Mousetrap, with the play, Hamlet, because the titular character has been tasked, by the ghost of his father, with exacting revenge on his father's murderer. The ghost tells his son, Hamlet, that he was murdered by his own brother, Claudius, the man who is now the king, Hamlet's step-father, and Hamlet's uncle. Hamlet feels he must make sure his Uncle Claudius is truly his father's murderer before he can take his revenge. Therefore, when a troupe of actors comes to town, Hamlet asks them to perform this play—with a plot that is similar to the story his ghostly father told him about his own death—so he can observe his uncle's response and ascertain whether he is truly guilty. As Hamlet says, "The play's the thing / Wherein I'll catch the conscience of the king" (Act II, Scene 2, lines 566-567).