Answer:
It reveals the order things happened in the past: the first action is, in this case, pointed out by the use of Past Perfect or Past Perfect Progressive and the second action or actions are written in Past Simple, because they happened afterwards.
Step-by-step explanation:
When we have two actions in the past, you have to use Past Perfect (when you emphasize a fact) or Past Perfect Progressive (when you emphasize an action in progress) to point out which action had happened first. The other tenses are written in Past Simple to point out that they happened after the one which had been written in Past Perfect or Past Perfect Progressive.