Ge 38:27 Now it came to pass, at the time for giving birth, that behold, twins were in her womb.

28 And so it was, when she was giving birth, that the one put out his hand; and the midwife took a scarlet thread and bound it on his hand, saying, "This one came out first."

29 Then it happened, as he drew back his hand, that his brother came out unexpectedly; and she said, "How did you break through? This breach be upon you!" Therefore his name was called Perez.

30 Afterward his brother came out who had the scarlet thread on his hand. And his name was called Zerah.

This story was recounted by Moses in detail to teach his original Israelite readers the importance, significance and privileges of being the firstborn in the family. Moses would never take the hassle of detailing the breach of Perez to be the firstborn over Zerah if it has no importance. The firstborn in Israel had the benefits of receiving the family blessing and the right to double inheritance. The idea connected with the firstborn ultimately points to the Lord Jesus Christ as the firstborn of all creation. God is training them for His coming.

Col 1:15 He is the image of the invisible God, the firstborn over all creation.