It is done when the company is not trading as the value of the sum of its parts. It allows each company to focus on a strategic direction of maximizing value of the each asset which may not be possible under the same company.
Spin out should be ASAP IMO as this BoD are having enough trouble getting value out of either asset.
Check when ASM was spun out of ALK. The ALK share price barely budged and ASM mooned.