The playwright must find new ways to surprise the audience, or he must find ways to make the expected ending part of the satisfaction of the work. One of the ways to do this is prophecy. Prophecies riddle the history plays, and part of the satisfaction they provide comes from dramatic irony. That is to say, usually the audience understands the meaning of a prophecy long before the characters do. Shakespeare often plays with expectation so that the limitations of historical truth do not translate into a boring theatrical experience.
Expectation also makes as see the past as part of a pattern.
A sense of destiny pervades Hal's character. He will become a great English king, and the audience knows it long before the characters of the play do. The men who wear the crown in the Henry IV plays speak of kingship as destiny. Henry IV is seen in this play as sleepless, anxious, fearful for the future. His duties as monarch and anxieties about the stability of his dynasty are sources of pain, not pleasure.