The product can change state. It can become viscous, interfering with the free movement of the springs and bellows. It can solidify, gluing the faces together or making the moveable components inoperable. It can crystallize and interfere with the moving parts of the seal. Corrosion increases with increasing temperatures.
Temperature causes materials to expand.
Seal faces can go out of flat, and pressed in carbon faces can loosen in their holder. Bellows vibration dampers can stick to the shaft sleeve, opening the faces. Some seal faces can be damaged by high heat. Plated materials and filled carbons are two such examples. If you take no corrective action one of the above will occur.