The motivation behind assembly theory is solving the origin of life and finding aliens.
The key idea of assembly theory is that the Universe cannot generate complexity outside of living processes. The complexity of an object is formalized as the minimal number of steps for making an object. If an object requires a lot of steps to make and is in high abundance, life is the only thing that can make it.
The conventional definition of life is a self-sustaining chemical system capable of darwinian evolution.