getServerSideProps fetches data on each request at runtime:

export async function getServerSideProps(context) {
  const { req, res, params, query } = context;
  
  const data = await fetch(`https://api.example.com/user/${params.id}`);
  const user = await data.json();
  
  return {
    props: {
      user,
    },
  };
}

Use when: Data changes frequently, requires authentication, or needs request-specific information.