It's "needed" to increase throughput.
The current head station is a head station meaning it ends here and then they do a U-turn and go back different direction.
Also it's planned as a fully digital station which is supposed to increase the throughput as well as the trains can be dynamically throttled when enetering the station.
39 trains per hour with the old station is still more than 32 per hour with Stuttgart 21.
Also groundwater, nature and referendum are totally meaningless words for anyone supporting this idea.
If I remember our guide correct, the current max speed into the main station is 50kp/h max vs S21 being at 80kph.
Because it's planned as the first fully digital station (meaning only digital equipped trains can pass due to signaling) it should be able to not require full stops or similar.
At least what the guide told us seemed all logical and dainty but it still is taking a ton of time and money.