When there is only a single action which a user can perform on an object, (such as a lever), a dialog system is not needed.
Within the scope of simulation interaction, the primary motivation for dialog systems is to allow the player to choose from lists of predefined actions to perform.
Dialog systems provide an inexpensive means of adding context and flavor to the setting, such as providing lore, backstory, and rumors.
Some story-heavy games (such as interactive novels) rely heavily on non-branching dialog systems to convey the narrative.
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I have no interest in making an interactive novel.
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A dialog system can be minimal and avoid natural language, but that is rarely employed.
Usually, if a game has a dialog system, the game designers take that as an opportunity to add a human flourish to its flow.