1) I'm not generally in the habit of prognosticating all of the reasons a corporation the size of Google tries to do something, especially not when there's an apparent and obvious win possible from the change itself. Minimizing the URL bar leaves room for other things on the page, including chrome extensions and, possibly, page content.

2) UI improvements are not "cosmetic reasons," they directly translate to ease and efficiency of task completion. As a UI engineer, hearing my category of work called "cosmetic changes" can be, quite frankly, exhausting. If someone were to halve the default font sizes on all browsers, would we consider that a mere "cosmetic change" or would it be a change that would have direct and immediate impact on people's ability to accomplish tasks?