They're a natural extension of the huge attraction of the draft itself, which is based entirely on projections and forecasts of college players. Of course they don't "mean anything" at this point. To say that we don't know how they actually turn out until we can see how they actually turn out is self-definingly obvious. But it's "done" every year and obviously generates a lot of interest among fans and analysts who like to speculate about college talent and project the overall direction teams might be headed in the future.