Important function of thick liner:

As the tooth preparation extends closer to the pulp, a thick liner or a base is used to augment dentin to the proper thickness range. Such a liner or base cannot harden by evaporation of solvent or water,  because it would not dry effectively.  Material used for this purpose hardens by a chemical reaction or is light-cured. In addition to thermal protection, liners are formulated to provide pulpal medication whenever possible. Two important aspects of pulpal medication are the relief of pulpal inflammation and facilitation of dentinal bridging for physiologic protection.  The materials (eugenol and calcium hydroxide) most commonly used to provide these two functions are not mutually compatible and cannot be used in the same formulation.