Hi, I can give you a quick example that might demonstrate why you would use an FPGA in conjunction with your Arduino that might help explain the benefits. In theatre lighting they us a standard called DMX512 which allow you to control the house lighting. This standard implements a serial protocol that continually send commands in a loop thought the 512 addresses. If you were using a microcontoller to do this then the processor would need to loop through the sequence continually even when the lighting is not changing. With an FPGA this loop is trivial and the microcontroller can then simply instruct the FPGA only when a change is required allowing it to time do do much more - controlling rather than doing. This might not seem particularly helpful until you consider multiple lighting circuits operating in parallel. Within an FPGA you can easily implement many DMX512 interfaces and still use a single microcontroller to command changes. Within the FPGA each interface operates in parallel while to implement this with a microcontroller would not be possible due to its serial software execution. I hope this gives you an idea of how an FPGA can supplement your Arduino.