Even if you don't care about privacy, every single page using bare HTTP can have HTML or JS injected into it by a third-party.

Certain ISPs, hotels and airlines do it to their clients, and the script kiddies messing around the poorly firewalled subnet of their college's AP do it too.

It would prevent one’s ISP from injecting advertisements in the HTML as has happened commonly in the past with US ISPs. I presume that has mostly fallen out of practice but one could well blame that on the near-ubiquity of HTTPS.