Another reason you might need a BC when flying with your kids: TSA PreCheck. Kids under a certain age (12, I think?) can go through the PreCheck line with their parent/guardian (assuming the parent/guardian got PreCheck). My daughter is 10 but looks much older, and I often have to prove that she's young enough to come through the line with me.
That's interesting. My son has been enrolled in pre-check since he was 5 and every time we get in that lane they stop us and say everyone, even the kids have to be precheck, and they NEVER believe us that he's enrolled until we show the pass (this is just to get in the entrance to the precheck line). Then as we walk through the line every agent we walk past repeats it yet again. We fly around the country a fair amount and get it everywhere (DC, Orlando, Seattle is really overkill, NYC, STL). I would love to fly where you don't get the 3rd degree.