It's to spread the voltage availability throughout the LED's. On long runs of LED's you well tend to get voltage drop's.
In my setup of 250 LED's I needed to power every corner and the mid section's of the horizontal bars. My first attempt resulted in orange-out's on the end of the LED strip farthest from the voltage source when I had a white screen up. (Low power = low blue and/or green)
Hope it helps.