I've already mentioned creating a simple stakeholder diagram as a tool to aid investigation and conversation. It is also important to create an architecture diagram for the same reasons. It is much easier to discuss how a system works and required changes if you can physically point to something and discuss it.
Do you really know what you have and where it is? Do you really know how it interacts with the outside world and with the infrastructure below it and the services on top of it? Legacy systems can be complex with a non-obvious structure.
There are an infinite number of ways to describe and document a system, from high-level overviews to low-level detailed descriptions of all components. You can show cross sections through any part of the system. The danger is that you will try to show everything on a single diagram and make it impossible to use as a guide to the system.