Why do cases have a perforated grill in the back?

[Bearmann]

You can see what I'm talking about in the second photo of this Cooler Master case review:

http://anandtech.com/show/9560/the-cooler-master-mastercase-5-review

With a front to back airflow plan, this seems like it would allow air to leak in from the back panel straight into the rear fan, short circuiting the front to back airflow effectiveness. Even with a different airflow plan, I can't see that this opening would ever be helpful as the airflow from the perforated grill to the rear fan doesn't seem to go over any components, except slightly the rear tops of the PCIe cards.

 

[DigDog]

a truly modern case would be SEALED with the intake and output being the only openings.
but that would require that not just the case, but most internal components be redesigned as well. the mobo i/o shield and the GPU for example.
in other words, cases are ventilated in the back because of history.

 

[squirrel dog]

that's to give bugs and dust easy access . I cover that with rolled foam used for air filters.Cut it to fit attach anyway you can(magnets/tape) When you do an annual or semi annual cleaning you will be amazed , no wait , shocked at how clean it all is .