There is a reason. The writer is trying to inject the exact tone of this conversation into the writing. If it were written plaintext, the reader could add their own emphasis to the words, and it might come out differently than the artist intended. When I’m making a more passionate point, I put a lot of words in italics—and I use a lot of M dashes. Because it paces exactly what I’m saying to make my point exactly how I want it to sound in your head.