Short Answer: Because the resulting video will look better on HDTV's. Long Answer: A progressive scan DVD player natively supports DVD's recorded in Progressive Scan as opposed to Interlaced only. In broadcast TV programs (Non Hi-Definition) a single frame of the picture is made up of 2 fields, each one showing every other line of the picture, these fields are shown at such a high rate (60 fields per second, 50 fields per second in Europe and other places that use the PAL system) that the human eye interprets them as being a single picture. Progressive Scan works by displaying both fields simultaneously at either approximately 30 frames per second, or 25 (Again depending upon location in the world). The advantages of a Progressive scan DVD player are lost on "normal" TV sets where it is virtually impossible to tell the difference, but on High Resolution Hi Defintion TV's, where upwards of 720 Horizontal lines can be displayed at the same time, if you were to view an interlaced source it would look very "jumpy". In order for you to be able to watch interlaced sources on a HDTV the source must be de-interlaced (through your reciever) and the resulting picture is only 1/2 of the normal horizontal resolution because effectively 1 of the fields is being thrown away.