You will not remember how things work
You may think you will always remember how your network runs, but as your network scales up, the less you will remember every single part. If you start documenting everything you have deployed, changed or upgraded, you will not have to remember everything in exact detail.
You will not have new engineers asking as many questions
It saves you time and effort if you have documentation for new engineers to read. They in turn will usually only ask you questions that are either not in the documentation, or they will ask questions for clarification purposes. These types of questions are great because it creates a better repository of documentation.
It is 3am, their is an outage in the network, and you are sound asleep!
Being sound asleep is a great thing. Being asleep means you have written the network documentation so well that the on-call engineer does not need to call you. The on-call engineer also will not have to waste time trying to understand the part of the network that is having the outage because they have read the documentation.
Your management will think highly of you
Managers enjoy documentation because it helps them understand what you have done, especially with project-based work. Management loves it when outages are resolved quickly because the documentation explained everything. You can then use your documentation as proof you have done a good job when you have your yearly review for a pay raise.
How To Write Usable User Documentation by Edmond H. Weiss
Handbook of Technical Writing by Gerald J. Alred et al.