Because the liner has no joints, it eliminates root ingress and groundwater infiltration, which are the two failure modes that end the life of Victorian clay drainage.
The pipe must be jetted and, where necessary, descaled first so the resin bonds to clean pipe wall. Skipping that step is the main cause of liner failure.
Relining is dramatically cheaper and less disruptive than excavation on deep runs, under driveways, or beneath extensions and conservatories.