Your cpu may run that game for days but do something else and fall over, prime95 basically computes numbers that are known already and sets your cpu computing them and compares your computed data to the known value, any variations usually show somethings not right. So while you may think your pc is stable you might do something to trigger that instability, also funny things may happen like lost data, corrupt OS, random reboots, lockups etc.. that will manifest itself over time in a unstable rig even though on the face of it it might seem stable. Prime does a few things that overclocking needs: checks accuracy and heats things up, stresses components and drains a bit of juice since its more or less a full load on the system, so weak points may show up. Superpi is a rather short test, so instability caused by heat or undervoltage etc.. may not manifest itself in the time taken to complete it.
