“In media res” is a narrative technique in which an author begins telling a story not from the beginning of it but from the middle of it or from a crucial event. In the case of Abraham Lincoln's story, that important event would be his famous speech "The Gettysburg Address." This technique carries the story from that point on, and then gradually presents earlier events through dialogue, flashbacks or descriptions.
The reader might become more attached to Buck, and if it was told from the trainer's point of view, it might change the reader's opinon on Buck, and make it more on the trainer's side.
