They differ only in the props.primary true / false setting and provide a simple way of having two styles of button.
This could be a simple way of differentiating navigation buttons (that can be pressed without fear of switching something on or off) and action buttons (that will switch something on or off!). Many of the components now have options such as "row" or "column" options from the component menu to save a little work later on.
See this for a little more on the topic: