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.