well when you start doing batches of videos it adds up quick, plus its good to be able have lots of them stored, that way you can clean off the drives in big batches and cut down on the maintenance, I do sometimes as many as 12-16 per day in a batch, so you have to have somewhere to keep them, scratch space to use while encoding them, then somewhere to put the finished video before it gets burned
mpeg2's in dvd format can eat up some space, recording video from two tuners means about 4GB's per hour incoming, before you know it after a few days youve got drives full of stuff
the new HD-DVD format means like 10+GB per hour per stream