You are confusing two different things. One is people who are not allowed to fly to and from US airports and/or barred from entering the US, and the other is people who are allowed to fly and to enter the US but who receive extra scrutiny when they do (inspections of their luggage, questions about their itinerary, checking of their credentials, etc.).

You might argue that the need to give extra scrutiny to those latter people could be waived when the computer systems go down, without much impact. See how far that argument gets you with bureaucrats who will be blamed in the event of a security problem, but are unaccountable for mere delays.

The other role for advance passenger information is that sometimes there could be a passenger of such interest that they are not denied passage but an individualized response is prepared for that passenger when they arrive. I don't know how often that happens but I would guess it's not zero.

But the best way, again, to understand these systems is that they proliferate because the people deploying them have not much incentive to increase the convenience to passengers (particularly noncitizens/nonresidents). Take money out of their budget every time there's a delay, and you'd see very different results.