Your engine requires a tremendous volume of clean oil to operate properly. If the oil filter is not changed periodically, the filter can become severely clogged, reducing the volume of oil passing through the filter and into your engine. Without this oil, your engine can suffer severe damage.
As a safeguard to oil filter clogging, premium oil filters contain a bypass valve which regulates oil pressure within the oil filter. If the oil filter were to become fully clogged, this bypass valve would open allowing oil to continue to circulate into your engine. However, the oil would be dirty, unfiltered oil that has bypassed the oil filter. Without that bypass valve, your engine would have sustained terminal damage.