I dug out this old thread from a website, that I used to visit often, about "ZGL's six northern campaigns (against Wei)". I wrote "Thanks Lu Kang your map is much better than novel's deion. By examing your map I can see that Jieting was a spot connecting An Ding Commandary and Tian Shui Commandary through a mountain range." in that thread. Tian Shui was under the control of Shu after the surrender of Jiang Wei. I think Jie Ting's importance is the supply route across the mountain into An Ding. I believe An Ding was also under Shu's control.
ZGL thus had minimum control over the areas north of Jieting (he was not there yet).
Zhuge Liang had three options with Ma Su up at Jie Ting before Su was defeated. He could either go up himself to hold Jie Ting, he could have brought an army down to strike Chen Cang, or to cut off Zhang He, or he could have stayed where he was (to bad map is gone). He stayed put it ended with very poor results.
Ma Su should not have been holding such an important location if the plan was to capture the western areas. If the plan was to draw the army away, he definately should have struck out more. The point of Zhao Yun was to draw an army off that way. ZHang He brought an army with a long supply line to Jie Ting (from Chang An). If Zhuge Liang had struck Jie Ting, the Wei army would be greatly streched and both Cao Zhen's and Zhang He's positions would be greatly threatened.