During his closing argument, Atticus does something that astonishes Scout and Jem. What does he do and why?

Atticus placed his watch and chain on his table. Then,"He unbuttoned his vest, unbuttoned his collar, loosened his tie, and took off his coat." Atticus is an educated and progressive man, he needed to show the jury he was a citizen of Maycomb, a Southerner just like them.