You may have numerous servers in production, providing core services required for smooth operations. However, when systems near their end-of-life, the transition to replacement servers can be daunting.
By documenting your existing server configuration; clearly establishing required services and long-term plans for growth / change / evolution, you are assured that a new solution will meet your current and future needs.
With rigorous documentation of all work performed, you are assured that subsequent / future work will be straightforward.