The idea is to focus all the light sources to the point at which you are sitting. Unfortunately this means there is only one optimal spot to sit, at the focal point. The logical conclusion would be to design a convex screen that curved in both directions.
The reason this is not done is that you then would need to be at the exact focal point to see a good picture. The thing is a gimmick some sales people thought they could sell.
I suggest that if people are not seeing a good image on their HD tv set they should visit the optician closest to them!