It would seem over 200 mph isn't useful, except the Boring company could change that. A private tunnel could enable such leisurely travel for only cars that had level 3+ autonomy safety features.
200mph top speed is just a consequence of the base abilities of the car, it does not really matter in practice, but the fact that the car is capable of doing it shows that it is really powerful and very aerodynamic. Besides far away niche uses like tunnels under LA and autonomy, in Germany you could drive 200mph if you find a good stretch in the early morning ;)
At this price point, the target market probably isn't prioritizing for usefulness / raw utility as a purchasing criteria.