The function of an input/output (I/O) device in computing is to facilitate communication between the computer and external entities, such as users and other systems. Input devices like keyboards and mice allow users to enter data and commands into the computer, while output devices such as monitors and printers present processed data to users. This interaction is crucial for completing tasks efficiently and effectively. Modern systems can also utilize peripheral devices that combine input and output functions, enhancing the overall user experience and expanding the functionality of the computer system.