The Flyswatter3's serial port provides you with an independent functional "USB to RS-232" serial device. The serial port is completely independent from OpenOCD on both Linux and Windows. You can use the Flyswatter3's serial port and never have to use OpenOCD or JTAG, or you can use it together with OpenOCD and have both a serial port and JTAG interface operating at the same time for debugging your target device.
For Linux, the RS232 driver for the FT2232 is part of the main kernel tree and is provided in most standard Linux distributions. In Windows, you have to load the Windows driver for the FT2232. Once the driver is loaded, Windows will assign a virtual COM port to the Flyswatter3's serial port. It operates just like a standard COM port. You can use the Flyswatter3's serial port on laptops or PC's that do not have a 9-pin legacy serial connector.