What conceptual role does the Kuiper Belt material represent in the context of solar system evolution?

The core material that failed to become a planet like Uranus or Neptune

One compelling interpretation of the Kuiper Belt posits that it represents the leftover building blocks in the outer solar system that never managed to aggregate into a full-sized planet. If the gravitational influence exerted by Neptune had been weaker, the concentration of material in that region might have allowed for the necessary accretion to form a fifth ice giant, structurally similar to Uranus or Neptune. Instead, Neptune’s dominant gravity scattered much of that potential planetary mass out of the system entirely, leaving behind the substantial population of KBOs, including dwarf planets like Pluto, that currently constitute the Kuiper Belt.