Remember: Congress has nothing to do with this set-up because NSC is considered to be part of the President's staff. Also: all it takes to change this organization is a President that wants to change it.
Opinions and info are going to go back to the Principal's Committee.
There are so many voices that sometimes it is hard to figure out what to do. Yet, how come the NSC ends up being so important to determine policy?
bypass parochialism, organization's culture
The NSC only has a little budget for its internal operation only. It has no budget for implementation. It is other agencies that implement the policies that the NSC has decided. There is a bureacracy vs. bureaucracy check, since other agencies have to decide whether the NSC's recommendations/options are actually feasible.
Little or no oversight; Except where the President is concerned.
Possibly the biggest influence of the NSC over policy, is that they provide the president with available POLICY OPTIONS. The NSC can filter out undesired option, therefore taking them away from the decision-making process