Earlier quoted context omitted.
I didn't know that. Based on what do they print a boarding pass, or perhaps rather: why don't they check {whatever the answer to the previous question is} at boarding instead of you having to hold onto a pass that apparently is a proxy for something else?
Speed. It takes time to look up a passenger on a computer. The plane will be delayed if they have to do that for everyone while boarding.
I'm just speculating but if they can simply do a name+DOB-based lookup at a counter, it seems to me like the boarding pass thing might just be for historic reasons where people would feel weird if they are suddenly tracked based on personal details instead of a ticket