Its black Hat SEO Technique. 100 % chance to banned yr site by Google SE. one type of Unethical SEO. Watch out best example: http://www.applicationdevelopmentindia.net/ this is doorway page site.
It's under Black Hat technique; used to trick SE spiders into believing the page is highly relevant for a given keyword, and thus warrants a high position on the list. Doorway pages are single-page websites that link to one or more other sites
as what they say its a black hat technique, usually done for the purpose of high ranking in search engines Its totally created for spamdexing,