The styles show here are for true buttons, which use either the <button> or <input type="submit"> HTML tags. They are typically used for things like form submissions, or to trigger some kind of action on a page. For consistent user experience, the styles that can be applied to real buttons are limited.
There are also styles that can be applied to link that make them look like what we think of as buttons. Those can be found under Atoms > Links > Button Styled.