The law as written makes it illegal to break DRM even for your own use, this exception makes that legal. But also, it doesn't affect only users, but toolmakers. As things are, tools to break DRM are considered circunvention devices and sometimes get taken down by ISPs because of it, people who develop or research them get in trouble for it, and so on. So the exemption has also the use of making that sort of work less risky.