The #3543 Motor Driver and Power Distribution Board for the Romi Chassis is intended to make it easy to get your robot up and running quickly. Along with a power switch and built-in motor driver, the board has a powerful switching step-down regulator that can supply 2.5A continuously at 5V or 3.3V, and makes it convenient to connect to the battery terminals. With all of the features built into the board, you essentially just need a microcontroller and any additional sensors you want.
However, the Romi chassis is intended to be a general purpose robot, and you could certainly use the chassis with your own electronics. If you want to use your own motor driver, I would recommend considering the #3541 Power Distribution Board for the Romi Chassis, which is less expensive than the #3543 board, but still offers a convenient way to connect to the battery terminals, a power button/switch, and easy access to various power busses.
By the way, when choosing your motor driver/controller, be sure to choose one that is appropriate for the motors. We generally recommend choosing a motor driver that has a continuous current rating equal to or greater than the stall current of your motors. The #3543 Motor Driver and Power Distribution Board uses two DRV8838 motor driver carriers, but if you are looking for a driver with an Arduino shield form factor, you might consider the DRV8835 Dual Motor Driver Shield for Arduino, which is rated for slightly less than the stall current of the 120:1 HP Mini Plastic Gearmotors that come with the Romi Chassis, but should be fine if you avoid stall conditions.