Purpose of Aiming screen for antenna...
The aiming screen is for zeroing in on stations that you can receive from your DMA or from other DMA. Using the aiming screen can fine-tune your antenna for the best reception of one or all of the channels you want to receive. For example, if you receive multiple channels in one direction (East, West, North, South) then you might want to aim in the best area of this direction in order to get the best signal quality overall out of those channels without having to move your antenna constantly by hand. If you are in the position to receive channels from more than one market (DMA) then you might consider the rotator option. The antenna aiming screen best comes into play with this option. Depending upon how far away the signal in, the slightest angle turn can give you anywhere between not enough signal and just enough signal. I have done this with multiple stations to get the best performance out of my equipment. For example, I can turn my rotator to the exact angle of where I get the best signal quality, signal strength, and signal-to-noise ratio. If that particular channel is around 100 miles away, then just 1 degree turn means the difference between watchable and not enough signal.
In my situation, the antenna aiming screen has been a great reference for my rotator since I can get channels from 3 DMA's (channel markets).