It simply allows the oil to bypass the filter if it becomes restricted / plugged, in an effort to minimize catastrophic engine damages. There is only a single spring loaded disc that becomes dislodged from excess pressure, allowing the oil to bypass the filter cartridge. I've never seen one go bad, as it's a very simple design, with one moving part. The spin on oil filter adapters usually do not have this valve, as the filter has the bypass incorporated in it, while the old style paper cartridge does not have this provision. Many people use the spin on filter adapters, and plug the port for a check valve, when using a high pressure / high volume oil pump. Hope this helps to explain the function of the check valve.