Larry Scott never explained why the preacher bench needs to be convex with a bulge in the middle. Does it just 'feel' better or is there another reason? I think I know.
He didn't say, except that it was way better than the flat surface preacher bench, in terms of results obtained. And he was living proof of those results.
My take on it, is that the convex surface allows for the natural hyper extension of the elbow joint, that allows for greater muscle use in the curl movement, including the negative. Would work that way too, for the "burns" he did, that were great for brachialis growth.