timing simulation
While you do timing clouse, you have to make sure that in all the operating conditions your ckt's timings are met and ckt works fine. so to ensure this , we do timing clouser in best case(worst hold check..i.e minimum time) and worst case(worst setup check). While you simulate in bestcase, you will have a smallest delays,(high Vdd, low temperature, best process). With this situation, you may encounter hold violations, so we simulate this condtions to make sure that even in the best conditions we are not facing any hold violations. Similarly for the worst case (low Vdd, high temperature and worst process so worst/highest delays). I this case we may face setup violations, so we simulate this conditions to make sure aht even in the worst case conditions we are not facing any setup violations.
Hope this helps you.............