Controlling the rate the beans feed into the burrs prevents them from crowding the grinding chamber and reduces the chance of a fractional grind. As well, starting the bean out cracked and giving a larger surface area for the burrs to do their job allows them to shave through the bean rather than crush them, reducing frictional temperature and preventing premature brewing of the grounds.
The Zerno design team extensively tested variable speed methods and found most flat burrs are designed to function in a specific range and increasing or decreasing the speed often led to unpredictable results/did not produce results that met their standards.
The Z1 prebreaker not only feeds the beans between the burrs consistently, it regulates the feed rate, controlling the particle size distribution and ensuring grind uniformity to appeal to your particular taste.