AuthnQuery on Synchronous Bindings

[Celsus Kintanar <celedor@gmail.com>]

Hello,

I have been studying the SAML 2.0 for quite some time, and cannot see
the applicability of the AuthnQuery over a synchronous binding.  For
SSO, the SP tells the client to authenticate at the IdP, then come
back with the needed assertions.  If specific information is needed
that the client must not hold, then the Artifacts are available. 
Having the SP gather the credentials from the client and going
directly to the SP seems to complicate things (furthermore, I cannot
see how the credentials can be placed into the AuthnQuery).

I guess what this means is that the AuthnQuery profile is not meant
for SSO.  Could someone please enlighten me as to what it is for, and
how it may be used?

Thanks,
Celsus