In the original article here, they discuss how the reason for this feature is that some people get shadowbanned a little too aggressively. "Banned accounts sometimes post good comments, software filters sometimes have false positives, and users sometimes flag things unfairly."
It seems to me the whole point of the vouch features is to have an easier recourse for other commenters to respond to people being shadowbanned (or otherwise dead for mistaken reasons, like tripping a filter or unfairly flagged by other users), rather than actually having to find the mod email and send an out of band message.
The problem is more complicated than that. Many banned accounts' comments aren't all bad. Unbanning them wholesale isn't an option if they're still going to break the HN guidelines. But killing all their comments wholesale isn't great either, if sometimes they post valuable things. So what we want is a mechanism that works at comment granularity rather than account granularity: let the good comments, and only the good comments, through. That's what this is intended to be.
It seems to me the whole point of the vouch features is to have an easier recourse for other commenters to respond to people being shadowbanned (or otherwise dead for mistaken reasons, like tripping a filter or unfairly flagged by other users), rather than actually having to find the mod email and send an out of band message.