What is the purpose of polyethylene encasement for ductile iron pipes?

The provision of polyethylene encasement avoids the direct contact of ductile iron pipes with surrounding aggressive environment. However, polyethylene encasement is not intended as a water-tight system. Initially moisture between ductile iron pipes and polyethylene encasement contains some oxygen which shall be ultimately depleted and this would put a stop to the oxidation reaction. This process leads to a stable and stagnant environment near the pipe and it provides a protective environment.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.