The amount of power transferred from point A to point B via transmission line depends on how much the difference between the power angle (voltage angle) at point A and point B. The larger the difference, the more power will be transferred. But, that difference has a limit for stability issue. One of the goals of power system is to transfer power from one location to another as far as possible, so we use long transmission lines. With long transmission lines, we have reactive power shortage problems, and stability problems, which need to be considered carefully. So, to tie up the two, we transfer power long distance, by increasing voltage angles. Recall the power formula from any power system analysis textbook.

thank you Jeremy Lin.it helped me.