The reason you don't get it is that taking away the functional part of onBeforeRequest() doesn't really help with privacy. Because extensions can, for example, still inject arbitrary javascript if those permissions are in the manifest.
What it really does is ensure that adblockers can't do heuristics, and instead have to rely solely on semi-static lists of urls. That's a nice outcome if you're a company that makes most of their money from ads.
There's not really a nice way to say that aloud though, so trying to make it sound it's a way of ensuring extensions honor privacy and security sounds better.