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The author uses the words usually and certain to emphasize that
The words usually and certain indicate that, for some products, the life cycle described in this paragraph is not applicable, i.e., the life cycle is not universal.
We know that the amount of success varies from product to product.
Since the author advocates a model of product life cycle, it can be inferred he or she believes this model has some use in prediction of success. The author uses these words to qualify (i.e., to add a reservation to) his or her statements about the product life cycle, in other words, to warn that they are not perfect.
Although we can infer from the sentence that some products do not succeed, the passage states that all companies conduct market research. Therefore, market research cannot be the reason that some products succeed and some do not.
The author is always an expert on the topic of a GMAT passage. In this case, the author does not want to make any sweeping statements about business success, as not all businesses are the same.