Why does Emerson most likely devote so much time to John Brown's family history at the beginning of his speech?

to show that he was in many ways no different from the audience.

Explanation:

Emerson's main purpose was to increase sympathy for the plight of John Brown. The way in which he encouraged this was by giving information about Brown's private life, such as information on his wife and children. He believed this would show that Brown was not a criminal or a mythical figure. Instead, he was just a common man who had ideals and wanted to fight for them.