"User review" sites are another popular way. This is a big problem in the travel and tech sectors, where some of the million-plus-page-sites' pages have useful content but many are simply empty vessels with a keyword and an invitation to "add your review."
Yeah, those things are a plague on the web.
The site I'm building has 10,000+ pages (each corresponding to a species of insect, rather than a product) but the 9500+ which contain no real information are hidden from the public interface. The administrator can browse the whole thing, and a page becomes live only when real content is added.