The fourth advantage of democracy: rule of law. In democracies, the law prevails. Governments cannot do what is not authorised in law. Retribution cannot be brought down upon citizens that is not sanctioned in known law and managed through due process. People do not live in fear that someone will come knocking in the night and take them away. Property has legal protection and cannot be expropriated except by due process and with compensation. Contract is regulated by law. Public policy, policing, surveillance, land management, punishment – none of these are at the discretion of the governors. Citizens have protection and predictability in life and business. In short, there is rule of law.
Under autocracy, the rulers are above the law, not the law above the rulers. There may be rule by law, but not rule of law. One reason a regime needs to be autocratic is that it cannot prevail with rule of law.
Rule of law is not impossible in political systems that are not democratic. Hong Kong has until recently benefitted from a rule of law regime, including with an independent judiciary and freedoms of speech, information and assembly. That, however, proved intolerable to the autocrats in Beijing. Since Hong Kong liberties were not embedded in robust democratic institutions, they could be wiped out the moment the men in Beijing decided to take control. But rule of law without democracy, if not impossible, it is very unlikely and unlikely to prevail. Democracy without rule of law, on the other hand, is not possible.
For more detailed analysis, see How Democracies Live.
On China and Hong Kong, see The Perfect Dictatorship.
On democratic quality, see What Democracy Is For.