For example, identifying highly valuable site visitors[2] or understanding user intent.
Third-party services must collect their data from ISP's, phoning home software, or from scraping search engines.
Services that perform keyword monitoring only scrape a limited set of search results, depending on their clients' needs.
Keyword monitors do this more easily since they typically know what location their client is targeting.
Google releases its own data, but only in an aggregated way and often without assigning absolute values such as number of visitors to its graphs.