I am pretty sure it is for and I'm not being a smart @$$ but canceling noise( I know you already said that) but canceling noise for the person on the other end of the call or also for taking video. I know I have been In some noisy situations trying it out and the people I have been talking to have said they cannot hear much background noise at all. Also on the video side of it roofmonkey recently took some video at a concert first holding the phone normally and the sound was loud and distorted. Then took a second clip with holding his hand covering the normal mic and allowing sound to come in only through the NC mic and it was night and day difference. Way clearer using just the NC mic. You should ask him (roofmonkey) I'm sure he probably has the videos still or they are on YouTube and you could hear the difference. Sorry for the long winded response.
The NC mic is for both you and the party you are speaking with. It samples the "room tone" and uses what it picks up to filter that noise out compared to what the mic at the bottom of the phone is picking up. That is why even in a noisy bar or sporting event, the person you are talking to can hear you perfectly. It's also why you can hear them a bit better than without it.
They have NC headphone for range shooting and what they do is cut the outside noise when it passes a certain level, like from a gunshot, yet allow you to hear someone speaking to you just fine.