`disk formerly known as the Kuiper belt'
was predicted because it was needed as a source of comets (Fernandez).
Particularly, it was needed to explain the inclinations of the Jupiter-family comets.
(Duncan, Quinn, & Tremaine)
Median inclination only 11
Also, it was needed to explain the origin of Chiron.
Dynamical lifetime is very short (Oikawa & Everhart).
The basic idea is:
Objects slowly leak out of the Kuiper belt. (Levison & Duncan, Holman & Wisdom)
They get scattered by the planets.
Some enter the inner solar system
Continued study of this process led to:
prediction of the scattered disk.
estimates of physical lifetimes of JFCs.
ages of giant planet satellites' surfaces.