They need to have this anyway because they never know what might happen to that person. They could say tomorrow they have another job, or as my old boss used to say "got hit by a bus" and died. You'd have no recourse then, so you better have redundancy.
Of course, every job I've ever been in has been terrible about redundancy, and when people start leaving, usually it's the people with the most domain knowledge, and so you get a big gut punch right away, and then it snowballs as more work and responsibilities get put on other people and they end up leaving once they've had enough or found something better, and it just keeps increasing until you're basically totally screwed but hey, you're saving money by not having to pay all these employees anymore, and that's all your stockholders are going to see for the first year until your company completely falls apart.
