I took my spare tire and holder off. I was wondering what the vent thing was for that's in the middle of the tail gate?

 

Air pressure. You ever notice how it's kinda tough to shut the doors with the windows sealed and rolled up? And when you've got a hardtop, it's a lot tougher to shut doors with everything sealed. Well that little vent helps with this. Without it, the doors probably wouldn't shut. Really shows how airtight the cabins can be!

 

also relieves the pressure a little bit when driving with the top on and the windows down

 

With my hard top it is tough to shut the doors, but with my soft top...not so much. I replaced the header, did make it tighter but still not enough to kick the door back or inflate the top when shutting the doors.

 

it has slots cut in it and does the same thing as the factory one.

 

The rear vent's function is to ensure flow-through ventilation. The air comes in the front vent, and exits the rear vent.
Vehicles have had flow-through ventilation for many years.

 

I've noticed much greater header wind noise with the soft top up (doors on and windows in) when I have stuff in the back blocking the vent. And I have the "new header."

 

Yeah, on sedans the exit vents are sometimes visible on the C-pillars but usually the air exits the cabin through vents on the rear parcel shelf or to the sides of the rear seat and into the trunk, and from there out of the vehicle through concealed vents around/behind the tail lights/bumper/licence plate etc so we're often not even aware of them. On the JK with the spare removed, they stand out like the proverbial dog's gonads.