TLS allows the browser to detect content (like scripts) injected into original web pages by (potentially) malicious third parties. Or at least by third parties

TLS allows the browser to detect content (like scripts) injected into original web pages by (potentially) malicious third parties. Or at least by third parties who aren't using certificates issued by roots explicitly trusted by the browser software. Before the use of HTTPS in the early 90s, it was not an exceptional condition for third parties to insert content (ISPs inserted ads, libraries inserted cookies, BGP hackers just looked at traffic). One of the reasons SHTTP and HTTPS were invented was to allay fears by banks that entering credit card numbers on non secure links would increase the rare of fraud. But the main reason (I believe) we use HTTPS on sites that don't take CC#s today is because we cannot enumerate every potential risk. Because we cannot forsee the risk, doesn't mean it doesn't exist. In other words... "better safe than sorry."