Some of the managers I was the most fond of, turned out to be terrible managers. But it wasn't until they left, and I was put under a competent and professional manager, that I realized it.
Employees are just cogs in a big machine, essentially. They are reliant on the ones in front of them to provide them with good information to work with, so they can do their job, and pass their work on to the next person in the chain.
As a "unicorn employee" myself, I have fallen into the trap of irreplaceability a couple of times.
The problem with being irreplaceable is that you can't grow, get promoted, take vacations, or leave, without everything going to shit. My boss and I actually did the math, and if I walked out tomorrow, they'd have to hire three people to replace me.
This is a VERY weak point in their structure. I get eaten by tigers on the way to work tomorrow, and they'd be screwed for 6 months. If I get promoted, then the company needs to hire at least 2 people to fill my shoes, and cost of operations goes through the roof.
So we have spent the last year making sure I "Only do what I can only do" and offload all the other work onto other people in the organization who do those things.
This reduces the company's exposure if I ______________ tomorrow, and makes it easier to find someone with the specific narrow set of skills to fill my core role.
Perversely, it also protects me, since there are now 10 other people in the company doing small parts of my job, who all need to be in tune with me and my work and rely on me to make sure they meet their goals that are pinned to what I do. This makes me key to many people's success, and gets me and my contributions noticed much more. Before, I was working my ass off, all alone, and nobody saw how I did what I did. Now I and my work show up in quarterly reviews for 10 other people across 4 other departments, increasing my visibility and the perceived critical nature of my work.
All because my manager wanted me to be able to take a vacation and not be a single point of failure in the system.