It can be used to subvert the same origin policy and content security policy.

If you see a script tag with the URL bank.com/evil.js, the browser shouldn't assume that the bank is actually hosting evil.js. Even if the hash matches, the content might not be there.

The bank might be using a content security policy to minimize the damage that an XSS attack can do. It only allows script tags from the same origin. However, now an attacker just needs to load evil.js with a particular hash into the cache, and they can create the illusion that the site is hosting it, without having to hack the server to do so.