I've taken it off a few times and I can't see what purpose they might actually serve. Most of that mesh that is under them is solid anyway and not mesh.

Why are those vents there? Is there an actual engineering reason for them? Or is it just looks because cowls are supposed to have vents there traditionally? Please, enlighten me on this. It's an important thing I need to figure out for a project that I will have to post about in another section.

 

Keeps debris from filling the cowl and risk blocking the air intake above the blower motor would be my guess. My Mustang has a fully vented cowl grille and the cowl stays clear for the most part, and i park under trees. A couple smaller leaves get thru but not enough to cause problems. However, if you live near a farm or woods and want to open yours up, you might want to use a mesh of some sort on the backside as mice can fit thru gaps as small as 1/4".

 

If you are asking why there is a vent one one side and it is not sealed off like the other side then the answer is because that is the air intake for the HVAC system. When you have the heater or ac on and it's not on recirculating mode, the air is pulled in from that vent.

It's actually pretty nice to have the engine compartment sealed off from the HVAC air intake. That way it is not drawing hot air from the engine compartment like many vehicles do.