Why are the Kuiper belt and Oort cloud important?
Well, I don’t know quite what you mean by “important”, but what I can tell you is that large astroids that start off life in the Kuiper belt which later have their orbits perturbed into highly elliptical “cigar” shaped orbits are what we traditionally refer to as comets. Thus without the Kuiper Belt we would have no comets. Also, now that I think of it, the objects in the Kuiper belt and Oort cloud give us insights into the material that composes the building blocks of the solar system. Since we can assume that any object out there is essentially unchanged since the formation of our solar system, examining those objects gives us insights into how the earth used to be.
