Why is an earth wire incorporated in an airless spray material hose?

Because when paint passes through the tip of an airless sprayer it builds a static charge. When spraying an oil, this could cause a spark on an ungrounded system which could in turn, cause a fire.

Gas airless sprayers have a wire that needs to be attached to a ground prior to use. Electric airlesses must have the ground plug present whenever material is being sprayed.