I absolutely don't grasp the context you are providing at all. But the core seems to be:
Given these constraints, why would a leader hold a general election:
- Denizens of the realm do not require physical things for survival (food, water, shelter, etc.)
- Power is currently concentrated in a single person at the top (Satan)
An election can help to divert pressure from leaders, by providing legitimacy (whether it is an entirely free or fair election or not).
Can the dead die? If not, there is nothing stopping them from having a revolution any time. Death is typically a government's most final threat - without it, repression will be more difficult. This is especially true in your scenario where power is strongly concentrated on a single person.
Even if the dead don't have physical needs, they still have wants. They might experience social needs (acceptance, comfort, friendship), physical wants (comfortable furniture, food for the purpose of pleasure), status symbols (rank over others), or many other things.
If the dead in your world came from democratic nations (if Hitler is there, many of them will have) they may expect elections out of habit. Not having one may be reason enough to revolt.
Relations with Other Realms
Elections can improve relations with other leaders.
Satan might be the only game in town, but perhaps he isn't the only town. Are there other planes (Heaven, Niflheim, etc.) with rulers he might have to deal with? If so, an election (even an unfair one) might be advantageous. As a real world example, consider the many dictatorships (or weak democracies) that have unfair elections all the time.
Elections can weed out dissent.
One of the difficulties that anyone in politics faces is the lack of information. Politicians (or leaders, generically) often don't know who their enemies are, what issues motivate them, where they are located, etc.
Democracies solve part of this problem with an election. Each side puts their people on a ballot, so everyone knows who the players are. Each side articulates their issues, identifies supporters, and so on.
This happens even when elections are not free. In the Soviet Union (for example), elections were common. Many parties ran. And the Communist Party watched with interest everyone who sympathized with the other parties.