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.

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.