If you're using your Mac to make a phone call, and you're on hold for quite some time, you might want to continue using your Mac to browse the web or check email, right?
This turns out to be quite difficult with VoiceOver. Fairly often during a phone call, after bringing focus to some element in the app of your choice, VoiceOver fades out while reading the element text. This doesn't happen for every element, but it's enough to impede productivity.
As a workaround, you must move VO focus away from the element, then back, and hope you eventually get lucky and VoiceOver reads it without fading to zero volume.
FYI, Enable Audio Ducking is not checked.