that one doesn't connect the pin to +5V or Ground because you're making it part of a conventional circuit, but in order to expose it to a change in electrical potential that will trigger a HIGH or LOW reading... right?
correct
and for the aside about RF interference, etc.
see, the inputs are so sensitive (like micro-amps, maybe less, I am not digging through the datasheet atm) that a little electrical noise coming off of your LCD monitors backlight, the vacuum cleaner, or even some potential difference tween your finger and the circuit, can trigger a change, so if you want them to not give you garbage you gotta yank them one way or another and drive them by pulling the other way.
IE you have them pulled to +5, or logic 1 and want to signal a change you gotta ground it to ground or logic 0