Answer:
- To provide backstory about an important relationship
Step-by-step explanation:
The main idea of the narration is to describe how, under what circumstances, the narrator and Mitchell Thomas became such affectively bonded.
Although it is presented the event of "The two of us came into Mississippi together by way of East Texas", there is not really a detailed description of that, indeed, significant event.
The event is just mentioned, as important, because it is the start, the triggering event, of the important relationship developed by the narrator and Mitchell Tomas.
Thus, the purpose of the narration is to provide the background or the events that preceded and led to that important relationship, i.e. the backstory.