A comment on this question led me to think. Why do one party states hold elections? And why is voter turnout so important to them?
Elections give legitimacy. Even if there is no other option, if you can report that the majority of voters wanted the dictator to stay in power, it looks better. The dictatorship can then claim to the international community that they have a free country following the will of the people.
The goal may be to give their own populace the illusion of a choice to quell dissent. Look up super-pac delegates to see how they affect primary elections. If the party controls who both of the candidates are, it doesn't matter the result of the election.
In extreme cases, the goal is to identify dissenters for secret prosecution. Germany wrote numbers using skim milk on their ballots in plebiscites. They numbered the voters as they entered the voting facility. Then they could know who was disloyal to the government.
Edit: Someone asked for sources. Here is a book by Michael Salter that describes this. His source is the International Military Tribunal for war crimes after WW2. I originally read it in William Shirer's Rise and Fall of the Third Reich, which cites many sources.
I agree with Vincent's first point, especially the international angle. Elections is what respectable countries do, so the dictatorship holds an election. With "better" results than their democratic rivals. Other points:
- The population is expected to come for a public show of loyalty to the ruling party. Doing so in a public way inures them to other forms of obedience.
- Supposedly the North Korean elections serve as a census. Everybody is expected to vote, those who abstain need to be accounted for.