Answer:
It keeps the audience engaged in the plot by suggesting more potential conflict in the story.
It creates suspense as the reader wonders why Friar Laurence would utter this warning.
It provides a clue that something tragic will happen to the lovers later as the story progresses.
Explanation:
The warning is "violent delights have violent ends," if I remember correctly. This creates a suspense that keeps the audience engaged, casing them to wonder and theorize about what is to come (and providing clues about what is to come is essentially what foreshadowing is).
