A user gets added to "jira-users" after being added to a a custom group

[Svetlin]

I added a user to a group (which has the "Team members" role) and additionally added him with the role “Delivery Manager“ via the People > Add button. Somehow, he ended up in the jira-users group for which I get criticised. 

Can you elaborate why is that a problem? E.g. what potential security problem being a member of "jira-users" poses that for example the "Delivery Manager" role or "Team members" role do not? 

[Svetlin]

I figured out a membership in "jira-users" automatically grants the "Bulk Edit" permission. Does anyone have an idea why is this so? 

Does that allow a user to bulk edit issues in any project on the instance, or just the project where they are added? 

https://www.atlassian.com/software/jira/guides/permissions/overview#what-are-users-and-groups