How about this:
The wheel/tire is a rotary inertia carrier. It helps with impacts when you strike a rock, a tree, or just something really heavy. The "weight" helps carry the mower through its arc. This saves the pto drive from the shock of the same impact forces as long as the gearbox can take it. Same as a rotating weight on a baler to smooth the plunger strokes as it goes through some wet or tough hay. This is most effective if the cutter load is periodic: like a pendulum deal. Normally there would be a shear pin in there, too, just to prevent a little too much rotary assist. This can be a littler harder on the pto clutch, too, because the engagement takes more time to spin up the shaft. It just boils down to an energy absorber. Just like the flywheel on an engine smooths out the power strokes on a crankshaft. Better if heavier. Must be balanced pretty well, too or it will wear the shaft bearings. Looks to me like someone was breaking a lot of u-joints at the coupler or twisting the splines in the shaft. So much for spewing....
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