Hi Amar Deep,
The only network devices we authenticate to are Cisco IOS or ASA. Check the following KB article for a list of commands we run against them:
The above KB should also address your question about permissions.
About the differences, that's a big high level. With credentails we can tell things like Patches and Hotfixes that have been applied. Without them you still get great info like various DOS, Data leakage, server injection, etc. vulnerabilities.
I hope that helps!
Thanks for your reply. This was exactly I wanted to know.