I am not sure I see clearly where the wood ends and the foam starts in your pictures. In any event, technically you as a boat owner don’t have to use foam. Factories have to use a calculated amount of foam in boats under certain length to comply with US Coast guard rules and regulations.
In general, foam is used to provide flotation (keep the boat from completely submerged under water) and/or to provide a form (shape) to fiberglass on.
In your project, I am guessing for cost cutting reasons and/or to meet the minimum amount of foam required, they decided against using all wood under this rectangle.. Instead, they used wood under the engine mount and filled the rest with pre-shaped piece of foam and fiberglassed over both to form that rectangle engine mounting platform.
If I were in your position, and this is just me, I would not repeat this design. I would feel more comfortable with wood all over that rectangle. With that said, obviously nothing wrong with the design, it served the boat well all these years.