It's for extra security, for example in cases where there may be a keylogger present, like with a public or compromised computer, or for other possible compromising scenarios where one may want to avoid using the keyboard. Did you read the documentation?

keyloggers come in all shapes and sizes. there are software ones which you mentioned, and then there are also the hardware ones that get plugged in between your keyboard and the computer's PS/2 or USB port. they are easy and cheap to obtain and install, I would keep this in mind if you are using public computers.