Creating root collections
Root collections are those collections at the very top of the collection tree - usually these are organized by an administrator to a structure matching how you work.
Because of this the creation of root collections can be restricted to only Users that are in a User Group that has the role can create root collections
Tips
If you have user's that don't require creating root collections, but you want them to be able to create collections put them in a User Group that has "Can create collections" but not "Can create root collections", then make the root collections that are needed with an ACL that allow that User Group write access.