As search engines are looking to present useful content which is informative and well-written, if they come across irrelevant, tired or duplicated copy it only stands to reason that these sites will not rank as well.
In the same way, should the category or product pages on a site feature too few words, this gives them little incentive to feature that content in the search results. Check out the pages on the site of any successful online retailer noting how much content they devote to each page.
The content that appears on a site does need to be unique in order for it to rank well. Millions of pages are thrown out of Google’s index each month due to the fact they contain content which is exactly the same or almost a duplication of the information already stored by the search engine.