Because you have in machine storage and you can edit all your captures in machine when you feel like it, watch em, burn em whatever. The bigger the hard drive the more you can capture and deal with it later, it's just so much easier ande convenient for everything and you don't even have to burn a disk unless you want to keep it for later after the drive is full. It's like a poor mans Tivo or DVR setup.
The bad part is most systems are digital now so soon the tuner and all that will be pretty much useless and many people are into HD as well, like myself. If you have a use for it and it still works in your area it'd be great, but if you want to get some money out of it I'd sell it soon before you miss the usefull demand for it.