The purpose of the preliminary examination in the UPSC Civil Services Examination is to serve as a screening test to filter candidates for the Mains examination. The prelims consist of two objective-type papers that test general studies and aptitude. This stage helps assess candidates' basic knowledge, understanding of current events, and analytical abilities. Only those who qualify the prelims are eligible to appear for the Mains, making it a crucial step in the selection process. It also sets the groundwork for the kind of comprehensive knowledge required for subsequent stages.