While you can hand-code the HTML for a submit button in a form, WordPress has a nice function, submit_button(), that can drop in the correct code for you.
For a recent project, I wanted to restyle some buttons to make them look like links, like on the posts page, rather than buttons. While you don’t NEED an special class to pull this off, it certainly makes it easier and neater in the CSS. I thought this would be achievable in the $other_attributes, but the $type parameter accepts any string, not just the WordPress defaults.
Basically, whatever you put there will be added to the class attribute of the button and you’re good to restyle the button however you want using your custom class.