Ill agree that from 15' away SD looks nigh on the same as HD, on a small screen.
But to quantify just a little, normal TV or DVD create the image on screen from around 250 thousand dots max. Full HD uses over 2 million dots, at least 8x the data on the screen.
Try using a PC on a 480 line (SD) screen, it'd be a blur and your 'start' button would take up 1/3 of the screen. We've all been using computers that carry around double to 4x the quality of SD TV the last 15 years and we wouldn't manage without them.
But to quantify just a little, normal TV or DVD create the image on screen from around 250 thousand dots max. Full HD uses over 2 million dots, at least 8x the data on the screen.
Try using a PC on a 480 line (SD) screen, it'd be a blur and your 'start' button would take up 1/3 of the screen. We've all been using computers that carry around double to 4x the quality of SD TV the last 15 years and we wouldn't manage without them.