[chousuke]

Leaving aside all the other benefits and even if you never need to rebuild your system, having some sort of IaaC automation in place allows for extremely powerful change management. When your system is defined as code[0], change over time can be reviewed with a "git log -p", which definitely beats searching through ticket comments or ad-hoc documentation and attempting to reconstruct the history of change.

It's a no-brainer nowadays that software should be developed with version control. I don't see why infrastructure should be treated differently.

[0] Ansible playbooks are code, no matter what some people may think. It's a declarative'ish programming language with a silly syntax.