What is the purpose of the metal platter-cover all modern hard disks seem to have?

[LMF5000]

I'm referring to the thick metal plate that covers up a segment of the platters where the heads do not access. All the old hard disks I've taken apart over the years haven't had one, but every picture of a modern hard disk I've seen shows them with this component.

Here's a picture of the part:

So, what does it do?

Curiously, modern 2.5" hard drives don't seem to have it either - just 3.5" ones. I couldn't find any website that specifically mentions what it's there for. I have a few ideas, but I don't want to influence the answers. Suffice to say though that if the HDD would function just as well without the plate, the manufacturers wouldn't have put it there... so it must do something important

 

[SergeC]

My guess would be to minimize internal air movement, as there is more open space in a 3.5" drive. Small eddies would affect modern drives more than older ones, as the density has gone way up.

That said, it's just a guess.

 

[VirtualLarry]

it smoothes the airflow over the platters and reduces turbulence, allow safer, closer, head flying height.