samba - Feb 2011 - use of passdb backend in samba active directory mode

Dear Jeremy and samba team, when I move my nas to AD mode I see the smb.conf as
shown below.
what is use of passdb backend here ?, when anybody try to authenticate to samba
it uses the passdb_winbind.so and contacts the password server 10.32.125.116 to
verify login/password.
attached complete smb.conf
----

realm= EMCSOHO.LOCAL
password server= 10.32.125.116
security= ADS

passdb backend= smbpasswd
-------

Thanks
Suresh