Why is propaganda an important tool of totalitarian dictators?
It is crucial to control people's minds.
A totalitarian state is based on a complete hegemony of the state over the individual. To fulfill such a purpose it is important to control what people think in order to crush any type of dissent.
This is why censorship is also typically used by totalitarian regimes for similar reasons. Freedom of speech and thought is not compatible with any kind of dictatorship.