I bought a 2016 Rubicon that already had 37's with 20" Fuel Wheels and I noticed that every wheel has two valve stems. Why? What is the purpose?

 

in some cases they use two so you can inflate/deflate while using a pressure gauge to monitor tire pressure. may be that

 

more convenient to inflate and deflate. especially if one stem is buried in the mud.

 

That's what I thought it may be used for.

 

This is from Tire Rack...
That's for bleeding off the regular air when you fill them with nitrogen (a very common thing in Japan racing circuts). You fill through the bottom and bleed off from the top valve. Not only that, but they look cool.
Grant
The Tire Rack

 

^^^^^ this

 

It's so you can install Rimrock Mountain Supply Rapid Air Down Stems (RADS) on one side

This lets you have a rapid deflator mounted on the wheel all the time then you use a gauge on the normal side