Security is about being paranoid in imagining what different actors can do with varying levels of access. In this case, MITM is not always easy, but once they have access they can do bad stuff. As a webmaster, an attacker with this access can break your site by messing with your users.
By taking the simple step of using https, you can successfully reduce your attack surface and increase the difficulty of messing with your site.
By taking the simple step of using https, you can successfully reduce your attack surface and increase the difficulty of messing with your site.