If you don't document your systems, then you're relying on an oral history being told and retold. Over time this will drift, and the people telling those stories will find this becomes a large part of their job.
See also Write on Documentation Culture.
Documentation like all communication is lossy. It's important to automate processes when you know they won't change.
Documentation can be useful when you're trailing Business Processes where you want to trial a method or experiment with different approaches. When you know the best way something could be automated, go and make it happen.
Beware of Automation Bias.