Think of it like a supercharger on a car. The carb brings in cold, clean air, making the toke stronger (somehow) and smoother.
The way I get ripped off with shake constantly, I don't think I could smoke my shit out of a spoon. Used to get nice buds, but the current dealer sells mostly just shake in joints. Fuck joints. I just think shake will fall through the hole in the spoon (not the carb, the hole on the inside of the bowl where you put the weed..). Didn't know they made glass screens but I bet they're really fragile and/or expensive.
Yeah the glass screens are not really well made, meaning they have the tendency to fall out of the spoon. They don't break easily but they are expensive, about $6 each at the headshop I used to work at. If you are really worried about the weed falling through the bowl, which hardly ever happens, just leave a small piece of bud at the bottom. But even without the small piece of bud, hardly anything ever falls through.