Account overview was a productivity feature.
I have 30 mailboxes, with decades of ISP experience.
When I checked email, multiple times per day,
I could see at a glance which mailboxes were active, and respond appropriately to the more urgent boxes. These have zero unread.
Less urgent boxes have advertising or notification email etc. and can be left for a cleanup at a later time.
Notifications demand the same amount of attention regardless of which box they’re from
My original explanation was here K-9 5.800 List of email accounts after startup - #90 by k9mailuser
Thanks