Without scheduled shifts, drivers will purposefully all sign up for the same shift to induce supply overflow. In this situation, drivers can game the flexibility of the system to collect a paycheck while sitting in their cars doing nothing.

Even if drivers don't game the system, there will always be natural imbalance between supply and demand. Surge bonuses can mitigate this imbalance, but scheduling shifts is the more economically efficient way to solve the imbalance problem (hence why they say schedule flexibility will no longer be a feature in a driver employment model).