when do you actually need message queue dead-letter queues?

Hey everyone, so we've been running Kafka for some time, and generally, things have gone pretty well. But lately, we're noticing some messages get held up, and then they just time out if our consumers can't handle them. This got me wondering about dead-letter queues (DLQs). When did you guys decide you needed DLQs for your message systems? What kind of problems made you realize they were essential? Are there any particular signs or limits that tell you it's time to set them up?