What it actually does
A downspout concentrates a whole roof plane into one stream. Landing directly on soil, that stream erodes a hollow, and the hollow then holds water right against the foundation wall. A splash block absorbs the impact and spreads flow over a wider area so it can run off rather than drill down.
Where they fall short
A splash block moves water perhaps two feet from the wall. On the dense clay under most of the Kansas City metro, water that lands two feet out does not percolate away — the clay sheds it slowly, and the discharge zone stays saturated. Saturated soil against a foundation is what produces the hydrostatic pressure that shows up as a wet basement.
They also move. Freeze-thaw heave, mowing, and the force of the discharge itself all shift splash blocks out of position, and a splash block sitting at the wrong angle is worse than none at all because it directs water back toward the house.
When a splash block is genuinely enough
Where the grade already falls away from the house at a healthy slope, where the roof area feeding that downspout is small, and where there is no basement or lower level to protect. On a detached garage or a shed, a splash block is a perfectly sensible answer.
Better options for KC clay
The step up is a buried downspout extension running to a pop-up emitter or a daylight point well away from the house — that physically relocates the water instead of hoping the soil takes it. Where ground water rather than roof water is the problem, a French drain is the answer. We will tell you honestly during the free assessment which of the two you actually need, because they solve different problems and only one of them is usually necessary.