Undocumented work becomes unmaintainable. A flow built cleverly and never written down is a liability the moment the person who built it is unavailable, and clients discover that at the worst time.
Credentials pile up across clients. Administrative access to a dozen platforms is a serious responsibility, and removing it when a project ends is the step that gets skipped by almost everybody.
Automations need a scheduled review. Something built two years ago against assumptions that no longer hold keeps running perfectly and doing the wrong thing, and only a periodic read finds it.
The platform sets the boundaries. Rate limits, field types and what a system simply will not do are constraints discovered mid-project, so recording them per client saves rediscovering the same wall twice.