Adafruit explains : “Since the Raspberry Pi does not have a lot of PWM pins, we use a fully-dedicated PWM driver chip onboard to both control motor direction and speed.

The Raspberry Pi bonnet can drive up to 4 DC or 2 stepper motors with full PWM speed control offering a great solution for robotics, DIY milling machines or 3D printers.

Of course, you can’t directly control a beefy stepper motor to the GPIO pins of a Pi. You’ll let out all the magic smoke. But you can wire it up directly to a stepper driver board.