In many of these blockchain systems, the 51% attacks only applies to money in motion. I can't move all of your funds out of 'your account' unless I have stolen your wallet, and if I have stolen your wallet I don't need to change anything else about the network. I can just spend on your behalf.
Lacking that, I can convince you you got paid for services and then claw them back by making the network change their mind after the fact. I can repudiate all payments to you with a 51% attack, by voting again on what transactions just finished.
Congress has rules about votes. If you have a quorum, everybody sticks with the outcome. If you don't show up, you don't get a vote. To make that work, you have rules. A quorum is defined and static. Obstruction is not allowed - no secret votes, no locking the doors.
It may be as simple as this: When my transaction is settled I need to know the health of the system. If some definition of quorum is met I know my transaction can't be hijacked. If the network was degraded a revote is possible. I need to wait for more confirmation before rendering services.