It's good for transferring code ownership and development culture to a new maintainer, though.
Writing very clear code is not just a skill but an investment in resources. There are diminishing returns (a couple of my employers have had a much lower pain-point than I do), and making it fairly clear followed by explaining the rest is usually more effective. Admitting a fault expresses empathy, and honesty, and sometimes inspires the other person to get invested by proposing a solution, and who wants code owners that aren't invested in the code? Besides, zero gotchas is perfection, and we all know what perfect is the enemy of.
In scaling up a team, the alternative to getting new people to high competence on existing systems is to let them become highly competent on new systems. Over time though the old people only know the old stuff and the new people the new stuff, and eventually nobody knows what's going on. People only figure this out after the old people have already left. Old problems that haven't been fixed don't get talked about every day so it may be six weeks or a couple months before you figure out John was the only one who knew some bit of information and he's happy at his new job, and Sheila, the only other person who knew, has been gone for 8 months. And the person who wrote the thing just showed up on LinkedIn for getting a promotion at wherever they've been for 2 years. I wonder if they'd forward my resume to HR?
I want the new guy to eventually be able to do everything I can do so that I don't feel guilty about wandering off to do something else, internally or externally. These people can also take over answering for old arguments that I'm tired of having. Being able to tag out on that sort of stuff does wonders for my sanity.
I can certainly do all of this without pairing, but a lot of the time it's much faster and more illuminating for both of us. Especially when I can see how they stumble. Out of the mouths of babes, as it were. Maybe my assumptions were wrong. Maybe there's a simpler way to do this, or at least a more consistent one.
Does that mean that I've occasionally been the Jim in "Why the hell did they lay Jim off?? We needed him." conversation? Yes, yes I have. But I can tell you, being laid off feels a lot better than going down with the ship. Survivor guilt, being the last employee who still knows how to do onerous tasks, wondering who'll be next, constant existential crises feeling like it's on you to save the ship... It's all awful, and much, much later you start to recognize the scar tissue. Lay me off with a nice severance while there's still some money and a sense of guilt left, please and thank you. If you don't see my value it's because you're too busy looking in the rear-view mirror rather than what to do next anyway.