I think it is just a personal preference thing. I am pretture that some people just like the way they look. There are lots of fasion things that people do that have no logical purpose but they feel it adds character to whatever they are wearing so I'm sure that's the reasoning.
It might be a style. Sometimes people have metal clamps on their suspenders instead so they don't always use buttons. Buttons or clamps simply make them a bit easier to remove which is a plus.
