You need to be able to read documentation in all its forms: narratives, diagrams, models.
You need to be able to evaluate the quality of systems, such as internal control based in part on documentation.
SAS 94 requires independent auditors to understand all internal control procedures.
Documentation assists in auditor understanding and documentation of their understanding
Sarbanes-Oxley states that management:
Is responsible for internal control system
Is responsible for assessing the effectiveness of the IC System
Both management and external auditors need to document and test IC System