My mistake in terminology, he did say, and describe, control joints as opposed to expansion joints, but the advice is still valid. Control joints, whether formed during the pour, or saw cut later, are placed to provide a weak area on the surface of the pad in order to control cracks that result from shrinkage, or (less common) expansion.
As the concrete cures it shrinks, if it shrinks past a certain point there will be cracks, hopefully those cracks will form along the cut line. If the cut is filled with sand a void will develop, since sand doesn't have any elasticity. If the void is large enough, the overlying coating may crack.
Filling the crack with material that does have elasticity, and continues to adhere to the sides of the crack, will lessen the possibility of a crack developing on the surface coating.
There is also the possibility that the concrete expands, due largely to temperature shifts. In which case the sand may be squeezed up and out of the groove, again potentially cracking the surface coating. A filler that remains flexible can absorb some of that displacement, minimizing the chance of a crack in the coating.
Various seismic forces (settling, earthquakes, etc) can also cause slab movement leading to cracks, and again joints can help isolate those cracks, and a flexible filler in the crack can minimze the chances of cracks in overlying coatings.
If the joint is large, say over 1/4", it's prudent to "v notch" it before applying any filler. The notch helps spread material over a larger area, minimizing surface distortion...
If the crack or joint is deep, you can save on the filler by using "backer rod" jammed into the void, then topping with the flexible filler material...
Note that with a "flexible filler" the flexibility is relative, it'[s not flexible like a stick of gum. For example Wolverine Coatings IntegraFlex 1921 cures to a pretty hard state, although it does remain somewhat flexible and will elongate up to 50%. So you can drive over it, pull your engine hoist over it, but it will adhere to the sides of the cut/crack and expand and contract absorbing the energy and displacement, lessening the possibility of cracks developing on the coating.
