The main purpose of using glycerin when filling teeth is to protect the teeth and filling materials from the impact of saliva on them.

First, glycerin acts as a spacer between the tooth and the filling material. During the filling process, there are usually some defects or holes on the surface of the tooth. Moisture and enzymes in the saliva may enter these holes, causing the bonding force between the tooth and the filling material to weaken. The use of glycerin prevents this weakening of the bond by effectively isolating the saliva from the filling material.

Secondly, glycerin can also play a lubricating role. During the dental filling process, the use of glycerin can reduce the friction between the tooth and the filling material, making it easier for the filling material to enter the small holes on the tooth surface.

In addition, glycerin can also be used as a temporary protective agent for teeth. Glycerin is used to protect the tooth surface from external factors, such as friction and chemical attack, until the filling material is completely cured.

In short, using glycerin can protect teeth and filling materials, and improve the effectiveness and service life of fillings. However, different types of filling materials and different situations may require the use of different types of glycerin, so it is recommended to use it under the guidance of a doctor.