Edward the fat, son of queen Victoria, was so gross that he could barely button the second button on his lounge suit jacket so he left it unbuttoned. So the style was born.

A gentleman should never button all button of his suit jacket or sport coat. It is rudimentary in menwear. And, always unbutton when sitting.

The only exception is the paddock suit which has relatively higher button stance. For that, a man would button all jacket buttons.