Suppose you've built a project, some sort of device that, from time to time, needs to have some data or commands entered into it. The first thing that comes to mind is to connect that device to the serial port of your PC but what if you don't wish to do that for some reason. You could use a keypad. If you only needed to input some numbers it wouldn't be too much of a problem to connect a 3x4 keypad. But if you need to be able to input the whole alphabet then it is more difficult.
This project describes an interface from a 40 keyswitch membrane keypad to a serial line.