For what purpose does Brooks employ the flashback technique early in Chapter Three?

In Chapter Three, March's flashback illuminates the way he saw slavery when faced with it for the first time. As March lays on the ground writing to his wife, he cannot stop his mind from drifting back to the time when he left Clement's estate the first time: he wandered, disillusioned, through the Southern streets, before finding his way into a Christian church. Outside the church, a slave auction was happening and March was dumbfounded that the Christians did nothing to stop the sale of their fellow human beings.