Imagine a static website that provides crucial information, for instance, some regulation you have to follow. Your enemy MITM attacks that website and sends you a different page, which gets you in trouble.
The certificate guarantees that the information you are getting is from the entity who actually has access to the server serving the website.
even if you have something like a "donate" link that's goes to a third party payment processor. a MITM could redirect that link the their own paypal collections page.