Serial ports on USB allow further use of existing devices, even when the computer doesn't offer serial ports.
The high speed serial port with the 128 byte FIFO allow high speed communication, even in heavy loaded systems. Unique features reduce the system load. The serial port also allows unusual speed settings like 500.000bps.
Model USB-COM Mini provides LED for serial Tx/Rx, while alike model USB-COM DB9 has no LED.