STAR smog is pretty CA's new way to smog cars that have smog issues in the past or are most likely to have issues with smog (ie. cars like ours). Pretty much the DMV designates your car to a specific location to smog you car and once your car is there they inspect the emissions equipment along with any other part that can affect emissions on your car so if you have "illegal mods" it is much harder to get the tech To let you slide past visual.
Now living in the wonderful state of CA only provides me a handful of STAR certified smog locations that can give you the "hookup" with a STAR smog, however for a much higher fee (look to spend +\-$200) than most smog tests. But like I said earlier, if you want to save yourself the hassle of taking off every single modification you have done, putting on the stock parts, then putting the aftermarket parts back on then the owner of the vehicle can do so. But in my case, I don't have the time to spend swapping out a countless number of parts, especially since I'm going turbo.
On a side note, even certain "CARB approved" modifications won't pass visual anymore lol