When a project or environment is reaching a certain size a good documentation is incredibly important.
I personally think that even for small projects a clean documentation is important, but I wont’t go into that now.

If there is no well organized documentation you will run into several issues:
– Working hours will get “burned” searching for information
– Passwords get missing, resulting in issues
– Key knowledge held by single people (“Well, working in that network you just have to know XYZ, you know now!”)
– New colleagues cannot get up to pace without help, again burning time
– etc…

I could continue but I guess you get the point.
The obvious reason for writing things down is the human tendency to forget things.
However the most important reason is that there are more then one person tasked with planning, enhancing, maintenance and fixing urgent issues.
When changes are not immediately and completely documented, the other people working in the environment cannot rely on the information they have.
This WILL backfire, trust me.

A good documentation should should be designed to enable a person unfamiliar with the documented matter:
– Get a quick overview over the complete situation
– Get access to the available data in a structured and searchable manner
– Just have to look in one place

What you should avoid:
– Different files that have to be searched like Version_pre_2016, Networkplan_old, see documentation XY
– Old data like a not documented change of the contact person
– Incorrect data like the wrong device models. If you are not sure don’t write it down!

A documentation should at least consist of the following parts:
– Textdocumentation
Network Diagram
– Complete documentation

Good would be as well the following conponents:
– Hardware Inventory
– Software (license) Inventory
– Clear defined On- and Offboardingprocesses