When mixed with another material it is useful to make them have more friction, for example making honey, or making Rigid/Wall less slippery. Counterintuitively, it also acts as a decent lubricant because it gets stuck where it is put and makes a layer between objects. There is a special material called Anti-Viscous. This is created by setting viscosityCoefficient in the Parameters to a negative value. This makes Viscous want to flow more instead of less, and is useful for creating some very strange materials.