Q: What is the nature/use of the secure waiting area specifically if it is distinct from the platforms?

The platform areas are intended solely for boarding and disembarking trains. The secure waiting area will be used by high-speed rail passengers to wait for their trains once they have passed through the security checkpoint.

Other rail passengers need not pass through security checkpoints, but nonetheless require a dedicated waiting area in addition to the platforms.

If we compare the rail station to an aiport, the train platforms act as the jetways do -- merely for loading and unloading passengers. The waiting area is analogous to the departure lounges at each gate.