Rashi says that this is to tell the praise of the tribes of Yaakov. It was the time of the harvest when it would have been easy for Reuvein to steal wheat or barley from someone else’s field. But even as a little child from a poor home with lots of children, little Reuvein only took things that were ownerless.
Earlier in the parshah Leah names Reuvein. Rashi says that his name has a secondary meaning to the one mentioned in the Torah. The name also means, “see the difference between my son and my brother in law Esav. Esav lost his birthright fair and square to Yaakov, and yet he always harbored hatred for Yaakov and tried to kill him. Reuvein lost his birthright to Joseph, and yet he was the one who tried to save Joseph when the other brothers wanted to sell him.
Reuvein was content with his lot, and did not pine after things that were somehow out of his reach the way that Esav did.
Another similarity I noticed is the way that the Torah mentions that Reuvein went out into the field. The sadeh. Esav was called from his youth an Ish Sadeh, a man of the field. Clearly the comparisons between Reuvein and Esav begin from when they were both young.
Esav comes from the Sadeh tired an exhausted with nothing to show from it. It was then that he sells his birthright to Yaakov for a pot of soup.
Reuvein comes from the Sadeh with flowers that he legally got for his mother who was in need of something to make her feel better.
The frequency of the word Sadeh is not a coincidence. It is clearly there to distinguish the difference between Esav and the children of Yaakov. Esav was written out of the legacy of Avraham, even though he came from the same womb as Yaakov.
The brothers sold Joseph and could equally have been written out as well. it is important to carefully note their actions so as to understand what made them different from Esav.