Lead sheath is the best cover and practice to protect the cable core from penetration of all fluids such as water, oil , … other contaminated water with chemicals that can be harmful for the cable and cabling system in two major concerns :
1- These fluids will damage the cable core, specially the insulation of conductors and finally age the cable
and make it out of service much sooner than normal life cycle of cable (Normally its min 30 years)
2- Penetration of fluids to cable core, may be extended to whole cable run and ingress to switchgears and
damage the electric parts and cabinets.