Buttons were used to hold two pieces of material together. They were also used to decorate clothing. Some dresses had decorative buttons down the front even though they used hooks and eyes to close the front of the dress.
Zippers had not been invented yet so even men's pants buttoned down the front.
Men's military uniforms had buttons which indicated the branch of the service they were in. A young military man might give a girl he was interested in a a button from his uniform as a token of his budding affections.