When you design a roller-type mower, you should end up with even pressure across the width of the 2-part roller, despite the fact that the engine is usually offset toward one side of the mower. To get even pressure, you need a corresponding weight on the opposite side. For safe, convenient handling - especially on cross-slopes - you want the mower's center of gravity as low as possible, which means just bolting a flat slab of cast iron under the platform opposite the engine would not be the best possible solution, though it would be fairly good. I guess one fairly neat solution would be to have a cast iron weight inside the roller-half opposite the engine-side of the mower. Because the mowers come in multiple widths, you could vary where you put the weight on the axle (inside the roller), to get the weight distribution you need.
Having said all that, I don't know the designer and I'm guessing what he/she was up to.
