Because it explains why the work gained popularity, and what the nature of that popularity was. Why are you reading it today instead of something else?

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There's no reason societal approval must be taught uncritically. Maybe I used a poor term. I just meant that some historical works are seen as great literature. And the values that society currently held at the time, and subsequently must be part of the staying power of a given work. Obviously other things come into play as well such as writing style and quality.

I agree with you and think the term should be "important." There are works most would agree are important but not everyone would agree are good or "great" in an approving sense, like Joyce, for example.