The US Constitution sets the legal setting in which all American businesses are based. It is relevant because you know that Congress can levy taxes, regulate interstate commerce, and negotiate treaties such as trade deals. Relevant for business, it also gives the executive branch (president) the power to establish the Treasury Department and other departments. It also establishes an independent judiciary, giving a constitutional justification for any legal disputes a business may find itself in. Finally, it clarifies what role the states have in regulating business and commerce.
Really, any regulation in the US has its legal roots in the Constitution. Whether that be complex banking regulations, anti-trust laws, how to settle copyright disputes, or declaring bankruptcy, any business action that involves the government involves the Constitution on some level.