Plenty of websites think they only have to secure app.example.com, while www.example.com can be left uncovered even though it serves www.example.com/login, because /login POSTs to the app. subdomain, and the session cookie is only set
by and
for the app. subdomain.
From a naive perspective, all requests passing private information are protected in such a scenario. But, of course, since the www. subdomain is uncovered, it can be MITMed and replaced with a phishing site. (A.K.A. a "spear-phishing" attack.)
This change somewhat fixes that scenario: the developer can no longer keep the /login route on the insecure www. subdomain; they'll have to serve that page securely (either by making www. secure, or, more likely—because it's lazier—just moving /login to the app. subdomain.)
Even though www. can still be MITMed to replace it with a phishing subdomain, that subdomain can no longer serve a login form itself. It would have to link to a "real" phishing FQDN (one the attacker controls enough to get a TLS cert for), at which point that domain can just be found and blacklisted by the browser vendors.
In a sense, it forces the attacker into the open, where the attacker themselves can be caught/blocked, rather than simply their attack being caught/blocked. (In other words, it forces MITM attackers into a situation more akin to current botnet malware-writers, where they must put up C&C infrastructure which can be traced back to them.)
Of course, this isn't nearly as good as just securing the www. subdomain; and it will force much more work (likely, doing said securing) on those who want to embed a login form directly on their www. subdomain's landing page. But, in the interim while we work on universalizing TLS, it will drastically decrease the value of spear-phishing attacks, just as spam filters drastically decrease the value of unsolicited bulk email ad campaigns.