Slab suction is one of the four main forces that drive plate tectonics.
This is how throughout the Earth's history there has been the ability to create super continents where all of the land mass has converged into one (for example, Pangaea).
This driving force is important when the slabs (or portions thereof) are not strongly attached to the rest of their respective tectonic plate.
Computing the slab suction force can give us predictions of the plate velocities by using viscous flow driven by the slabs and adding it to the shear tractions the flow exerts of the base of the plates.
In the lower mantle if you have a decrease in viscosity the flow will become much more rapid and increase the effect of slab suction and if viscosity in the lower mantle increases the effects of slab suction will decrease.