Because even dictators don’t have power from “god”. Power comes from a group of people and autocracies differ from democracies by the size of this power-legitimizing group (I strongly recommend book “Dictator’s Handbook”).
In Russia this group consists of oligarchs and army generals. And those people need “normal” people to not be against government to work without complaining and revolting. One easy way to increase that satisfaction is by running huge propaganda camping and then organising theatre of choice.
Then high election result, makes people not revolt (cause they “chose” the ruler), reassures oligarchs and generals that it’s a good thing that you’re the leader and finally you can tell the international community that you’re a democracy.
So there’s a few reasons for elections in autocracies