How many times have you heard the phrase something like “the system/code is the documentation, if you don’t know how to read it then you should not work on it?”. So you have no document for your system.
There is no substitute for documentation that is kept and maintained separate from the system itself. This has the same controls as the system should have with version control and access restriction. When combined with change records this provides a history of what has happened to the service.
This information becomes invaluable when diagnosing a system failure, introducing new support personnel to the system, having to recover a system from backups, scoping a project, understanding the impact of a change, listing assets, knowing license position, etc.