TVs are expensive.
Why spend $400 on a television for your bedroom or your den, when a $100 TV tuner card will give you the benefits of a TV without the clutter?
More bang, less buck.
TV tuner cards add functionality; using free PVR software from the Internet, you'll be able to record shows you like for future playback without paying a monthly subscription fee.
Back up that tape.
You'll be able to hook up your VCR and save home movies to your hard drive, then back them up to DVD if you've got a burner. No dedicated DVD recorder necessary!