Each port has the digital isolator. The computer and your target board are electrically isolated.
The 4 ports can work at different voltages and the ground is not in common.
The noise coming from your computer doesn’t enter in your target board, this can be useful when you are working with sensible analog circuit.
The voltage levels of TX and RX are the same of your target board. This means that it doesn’t matter if your board is working at 5V or 3.3V or 1.8V, the logic levels are always correct!