[gentoo-user] Please explain why this new 'emerge @preserved-rebuild' is good?

["Mark Knecht" <markknecht@gmail.com>]

I've had this come up a few times over the last few weeks. Every time
I've done the command the emerge has failed somewhere along the way.

Is this adding some value that I don't understand? What does it do
that using revdep-rebuild doesn't?

Granted, I have no way of knowing whether revdep-rebuild would have
failed or not failed so maybe it's an issue with the quality of what's
in portage these days but I'm not grasping why things are getting to
be more work for fewer good results.

Thanks,
Mark