A CARB sticker/approved part(s) are required in some states that are very strict on emmisions & will open your hood during inspection. Also if a cop were to stop you, he could fine($500) you for not having "legal" parts, but what are the chances? I aint worried here in IL, now if it was CA it would be a different story.
Im not sure on the exact reason for CARB approved intakes, seems like it would be better, but what the hell do I know. One thing I can think of is that the breather connections are not possible & that could be an emmisions thing. Also noise is a problem - only a certain dB is allowed.