I always print or get boarding pass on my phone for my wife and I.

My step son says he never uses or prints boarding passes at all, always goes thru with ONLY drivers license once he has checked in for flight.

Is this possible?

Why do they always ask for both id and boarding pass?

What am I missing here?

Or is my step son mistaken?

I guess this question is for all domestic us carriers but I didnt know where on Flyertalk to ask it other than in an airline forum.