An argument for a partner or a mentee comes in the form of a ten ton truck.
I had a manager tell me once to write down what I did and how to do it and keep it in a binder. I called it my 'hit by a truck' book. I wrote it, but I didn't put a lot of stock into it, until I got chicken pox in my early 30s. I was out of work for about a week.
Because I took the time to document what I did, my team was able to step in and take care of my job, and call me at home for any questions.
Part of Leadership is making sure others can take the lead.