K 9
As Bear said, leakage of air around blade tip results in degraded performance. In the LP turbine the blade tip shroud seal teeth wear down creating leakage. Also, compressor blading tends to pick up dust that sticks to the aifoil surfaces and over time builds up changing the aerodynamics from what is desired. In the HP turbine, residue also builds up on the blade airfoils, probably coming from the fuel burn in the combustor that changes performance/efficiency for the worse. Today, some airlines have their engines water washed to restore the intended aerodynamics.