There are a lot of misconceptions about CARB and emissions in general.
The EPA is cracking down on vendors that sell "emissions defeat devices". This can mean reflashes that turn off emissions controls, full ECUs, test pipes or O2 "simulators" that allow you to remove a catalytic converter without the onboard monitoring system figuring it out. But it also means anything that can increase emissions. Vendors that are found selling parts like these are subject to fines that can be in the millions. We are not willing to take that risk, as we want to be around for a few more decades to support Miata enthusiasts.
This means that Flyin' Miata puts a strong emphasis on emissions compliance. The best way to prove that a product does not increase emissions is to undergo the CARB "EO" process. If a product has an EO, then it is proven not to increase emissions. It also means you can use it on a car in California legally.
So that's why we get EOs for as many products as we can. But what about you? What if you don't have local emissions testing? That may not be a permanent situation, so it's a good idea to make sure that any parts you put on your car will have no trouble passing an emissions test.
Note that CARB approval is not a measure of quality or safety for your engine. It simply means the parts in question have been through tests to ensure they do not adversely affect emissions over the course of the test. The car could blow the cylinder head off the engine on the way home from the test, but it still passed. That's why we do a bunch of other testing to ensure good driveability and durability as well as the emissions testing.
If you have CARB approved parts on your car and you modify them, they are no longer CARB approved. For example, if you add a boost controller to raise the boost on a turbo kit, it is no longer legal for use on the street in California. Whether or not the inspectors will notice this is a different question, but be warned.