a)If you wanted to share your personal workspace to all the others give your credit card info, and SSN.
b)By default the attachments area in workflows is set to public.Anyways the attachments area in a workflow is only accesible thru the workflow context so does not have any security implications
c)if you wanted to make a document/folder area like a webserver(Use livelink as a glorified($$$) webserver) you can turn this on.Advantageous because when the request hits for this object the perm bits are obtained from dtree as against dtreeacl.