Science Homework Help

I’m trying to learn for my Biochemistry class and I’m stuck. Can you help?

Food entry to the stomach causes the stomach signaling G cells to release gastrin, a hormone with two functions:

1- stimulates the parietal cells to release extremely acidic juice.

2- stimulates chief cells to secrete gastric lipase and pepsinogen, which is activated to pepsin in acid. 

Why should this long process happen to have the active pepsin enzyme?  Or why should pepsinogen be present in our stomach, not pepsin by itself?