What kind of advantages or practical reasons might there be for a nation, even a very powerful one, to legislate limits to its power?
The leaders of the British Empire in the late 1600's and into the late 1700's might argue that a few limits on the central power of the national government would be preferable to clinging to that power and losing an entire set of colonies, fighting a revolution, and watching those colonies morph into the most powerful nation in the world, the United States of America. Similarly, the king of France and his wife, Marie Antoinette, in retrospect might have found the idea of loosening the reins a bit a more palatable alternative to being captured and beheaded as France descended into a horrific revolution of its own shortly after Britain lost its North American colonies. In other words, historically, many governments which have sought to maintain autocratic, dictatorial control have found that it's harder than it looks because at some point, some people will lose interest in going along with the status quo, whether those people are citizens who engineer an uprising from within, as happened to Czar Nicholas and his wife Alexandra, or frustrated nations tired of watching the neighborhood bully roll over one country after another as was the case with Adolf Hitler.
Victor Hugo once said that "Nothing is more powerful than an idea whose time has come," and in today's world of easy Internet access, social networking, instant text messaging, and 24 hour a day news programming, keeping ideas out of the minds of a carefully controlled populace is trickier than ever. Nations like China, not exactly known for commitment to civil liberties, have a much greater challenge now than ever trying to police internet usage and access to search engines like Google. If there were ever a time when it might be most practical and cost-effective to loosen the reins of power a bit, it is probably in today's world where the dissemination of vast amounts of information and ideas occurs instantaneously all over the world and is increasingly difficult to control access to.