why preferences are asked in upsc ias mains form?

sir i wanted to ask why preferences are asked in mains form when state cadres are allotted purely on the basis of roster system.....pls help. -- TINU

In mains form they want to know your preferences for the jobs.

fill it till last if you dont want to miss a seat in the ship.

Further they want to know whether you are interested in your home cadre ...
If your reply is no - its negative sign.