TV-out is so that you can use that big 27" TV you have as a monitor and impress all your friends

. It can also be used for demos and stuff like that, so everyone in the room can see what you're doing.
As for watching cable TV, what you need for that is a TV tuner card, with a coax or S-video in port. These aren't generally found on standard video cards; ATI's All-in-Wonder line is best known for this type of card.
Hope this helps!