more weight over the pedals and less over the bars works the core more effectively.
Actually it is more weight over the saddle not the pedals! And this weight comes from the hips and butt area! THink of it as a gentle stretch and the more the glutes are back the more the hamstrings are engaged which helps create balance between quads and hammies. This is virtually impossible to do in an upright position. I very rarely use an up right position as it is awkward and just not standard unless it is in a recovery mode for you back!
Watch road riders, you wont see many in an upright, it slows down timw and impossible to climb hills!