What does a tracking event processor's token allow you to do that a subscribing processor cannot?

A tracking event processor's token is a durable record of exactly which event, in which segment, it last processed. Because that position is stored durably rather than kept only in memory, two things become possible. First, if the application restarts or crashes, the processor can resume from its exact last position instead of starting over or missing events that arrived while it was down. Second, an operator can deliberately reset the token and have the processor replay the entire event history, or any portion of it, from scratch — useful for rebuilding a read model after a bug fix or adding a new column that needs to be backfilled from history. A subscribing processor has no durable position at all; it only reacts to events as they're published live, so neither restart recovery nor replay is available to it.