Code does not explain intent
A repository shows what the system does now. It does not explain why the architecture was chosen or which alternatives were rejected. It also misses sensitive business rules and the changes that can be made safely.
The minimum documentation set
Every product should have an architecture map and setup and deployment instructions. It also needs an integration inventory, data model and configuration variables. Add backup and rollback procedures and a record of important decisions.
Documentation that stays alive
Documents outside daily work become stale. Make their update part of the acceptance criteria. An API change is not finished if the diagram and operating instructions remain out of date.
During an incident
In the first minutes of an incident, the team needs to know who owns the system and what it depends on. They must find the logs, contain the error and return to the last stable version.
The handover test
Ask an engineer who did not work on the project to start the application, trace a transaction and simulate a rollback using only the documentation. Their blockers reveal exactly what is missing.