Clocking is black head SEO techniques that don't accepted by Search Engine, In which presented content to the search engine spider are different from presented to the user. it purpose to cheat user and Search engine spider.
Here is no any secret or specific reason why people use cloaking if you use to optimize your site well. Many people donít want to lose their site design or redesign their sites they decided to use clocking techniques.
Many people use clocking techniques to get more clicks and visitor on their targeted URL.