goSerial is an application that lets you talk to serial devices.
This type of application is often refered to as a 'terminal', because it mimics the behaviour of the terminal hardware that was used to communicate with servers and mainframes in the old days.
While only few people need to talk to server machines through serial cables today, there are a lot of other devices that use some sort of serial connection:
* Cell Phones
* Communications Equipment (e.g. Packet Radio)
* Industrial or Medical Equipment
Although most new Macs come without an old style serial port, many of them feature an internal modem that acts as a serial device.
You can also get serial adapters for the USB port or PCI cards with serial ports and use those to talk to external serial hardware.