when moving that area is a high pressure zone, so it's used for cold air induction. Same as reverse cowl hoods. For it to be very effective though you really need a shroud around the filter that meets up with the opening in the cowl.
Yep, you will have the radiator fan pushing air toward the rear of the engine bay so you don't want air pushing that hot air to the front and stagnating around the engine, it's why you often see a cold air box around the carb filter which attaches to the hole in the bonnet.
I'd say it's an access hole, when leaves, etc. fall into that grille they get stuck there and decompose, then when it rains you have moisture soaked compost threatening to rust out that area.
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