[Avodah] When was the Bris Bein Habesarim?

[Marty Bluke]

If you just read parshas Lech Lecha you would think that first Hashem
told Avraham to go to Eretz Yisroel, then came the story with Lot etc,
and then came the Bris Bein Habesarim. However, the Rishonim point out
that if we take a closer look at the chronology we see that this is
not true.

Hashem tells Avraham by the Bris Bein Habesarim that his descendents
will be in golus 400 years. Rashi points out that we were only in
Egypt 210 years and therefore explains based on the medrashim that the
400 years started with the birth of Yitzchak. However, the Torah says
in Parshas Bo that we were in Egypt 430 years. Rashi there explains
(again based on medrashim) that the extra 30 years is from the Bris
Bein Habesarim, in other words the Bris Bein Habesarim was 30 years
before Yitzchak was born. We know that Yitzchak was born when Avraham
was 100 years old which means that Bris Bein Habesarim had to be 30
years earlier when Avraham was 70. However, at the beginning of
parshas Lech Lecha the Torah tells us that Avraham was 75 years old
when he left Charan. This means that Lech Lecha had to be 5 years
after the Bris Bein Habesarim. Tosafos in Shabbos 10b makes this
calculation and says this is what happened. Avraham came to EY when he
was 70 and went through the Bris Bein Habesarim, and then he returned
to Charan for 5 years until Hashem told him Lech Lecha.

The question we have now is why is the Torah written this way? What is
the lesson we are supposed to learn from the way the Torah ordered
things? Tosafos points out the discrepancy but doesn't explain why.