When you request a website over http your ISP can intercept the request (this is how captive portals for free WiFi work, and the reason for the existence of pages like captive.apple.com to be HTTP-only). IIRC they can also insert whatever they want to on the web page, I believe there was a case a while back of ISPs doing that.
It really doesn’t, on a technical level.
You’re not sending them any data. None that they send you is unique to you. There’s no real benefit in encrypting it.
By not transporting via https you can not be sure about that, because you can’t be sure it’s them sending you the data.
An injecting proxy could add ads, or scams to the content.
It may not make a difference on the sending end, but it does on the receiving end.
When you request a website over http your ISP can intercept the request (this is how captive portals for free WiFi work, and the reason for the existence of pages like captive.apple.com to be HTTP-only). IIRC they can also insert whatever they want to on the web page, I believe there was a case a while back of ISPs doing that.