I have two usecases in mind for this. The most important one is that you have the situation that the Ministry sends you (ideally) a signed .kgrp file and says "Hey these are the Keys for Project XY". Even if they are already signed by "email@example.com" you don't want to mark "sec-com-mngr-xy" as trusted for all your employees.
So you as the trusted key of your organization or team can sign this keygroup with exportable signatures and hand it out to your team.
The Dialog to certify all keys should show a checksum over all the keys signed as I have a related subtask in mind for exchanging printed .kgrp files.
Update (2023-09-28): We have changed the scope of this task a bit some time ago. In particular, the idea to show some checksum for a group was removed from this task. See T6469#174361.