First-in-first-out (FIFO) queues support progressive state iteration.

They operate by processing exactly one event (per conceptual state) at a time, and if there are errors with the event, the queue is slowed down to give the event a chance to succeed before moving along. This style is appropriate for things that must happen in sequence, such as state combinations. Something like event sourcing likely needs FIFO because the resulting state is the combination of all events in order. State machines and other systems of progressive modification also likely need FIFO. Systems that store new states as entirely dictated by the events, maybe like a non-progressive ledger, will not benefit from FIFO because the latest state is always the source of truth, regardless of what came before.