the imparfait and the passé simple serve two different purposes. The former is used in situations where you either describe an ongoing, repeated action or a kind of setting in which the focus of the narrative takes place.
The latter, however, describes an action in the past which is completely contained in the past and is used to relate an action or the main thread of a narrative. Usually you'd be considering passé composé in this context, but in a higher register of the language (usually written or literary French) the passé simple is used.
-- Chris (not a native speaker).
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