When there are a lot of child publications and you need to do a rollout to specific child publications, it would be handy if you have the option to manually select which child publications you also want to publish/unpublish content. So when you tick 'Also publish/unpublish in Child Publications' in the publish dialog, a dropdown would appear where you can select/unselect certain publications.
Note: this was the second most voted for idea from the former SDL Tridion Ideas site (submitted by Reinder Wit); it got 26 votes there.
It is sideways related to another Idea submitted here: https://community.sdl.com/ideas/sdl-tridion-dx-ideas/sdl-tridion-sites-ideas/i/sdl-tridion-sites-ideas/publishing-set-per-default-also-publish-to-child-publication-per-publication