Credential roaming allows organizations to store certificates and private keys in Active Directory Domain Services (AD DS) separately from application state or configuration information. For more information about this feature read Credential
Roaming at TechNet Library. This article explains a particular scenario where this feature is used in Windows 7. This article;
Credential Roaming in the TechNet Wiki explain it too with Windows 8 and show some FAQ about that feature.
Roaming user names and passwords was a feature for Credential Roaming in Windows Vista. The feature was then removed in Windows 7. Therefore, in a roaming enabled environment only Windows Vista machines will be able to roam usernames and passwords with each
other and machines running other versions of Windows will not be able to. This is an expected behavior.