Your company grows up. What was a three person team doubles in size. You hire more QA guys. You hire contractors. And then you realize: "Shit. We probably shouldn't give everyone access to our production signing configurations."
Whenever there's a new version of Google Play Services or the Support Library you probably go through and update each line one at a time. Smarter developers will Find/Replace to save a few seconds. And even smarter developers will do this.
During his session at AnDevCon Santa Clara, Doug Stevenson threw out this tasty bit: Some people use the Support Library to get bug fixes to framework APIs that wouldn't otherwise be available until the next android release.