I believe the dish hoods were developed to help the wing guys keep the front of the car balanced as they slid the top wing back. The air mass under the wing is increased and there is less resistance. So the air flowing under the wing doesn't stack up and it flows more freely allowing more pressure on top of the wing.

They also found out(What Bob East found out years before) that the incresed air flowing above the nose also pulled air through the radiator opening faster keeping that air from stacking up. Bob's cars do that as well and they cool better than most of the one piece hoods. Spike has adopted a version of the design as well.

Personally, to me a dish without the front wing is just there for looks and cooling on a non wing car. The aero effect isn't that great, but the nose/hood combos with the sideboards closing off the downtubes.....that is a different story.