If you have a marketing webpage, you might have a link to signup or login pages. If you can hijack the index page you'll also be able to hijack the links.
Even if you don't have signup or login pages, a MITM attacker can add them. Or they could add a "buy now!" link with a convenient entry form for the user's bank details. The relevant question isn't what your page has that's so important, but rather what an attacker could make it have that would cause trouble.