Is this something people do when they start a new job?
I kept a text document in which I listed who's who and what's what, and what their relation was to the organization I worked for. Nothing too fancy, just names, organisations, contact info and a few keywords.
It's a useful exercise to do at the start of a new position for several reasons. For instance, you want to ensure you didn't skip or include the wrong people when sending out an e-mail. Or you have to attend, or organize, a meeting, so you want to know who's going to sit around the table and how they tie into the goal of the meeting is.
When the lay of the land shifts fundamentally, it's useful to revisit the exercise.
If you're in a purely operational role and your territory doesn't extend beyond your team or your office floor, there's not much point in doing such a formal exercise.
I've also heard of it being used in situations with difficult colleagues or excessive office politics. Of course, by the time you are writing a stakeholder management plan just to deal with your own manager and peers, you probably need a new job, but it can be useful to keep you going for a while.