when should you consider a message queue

So, I've been pondering microservices and how they talk to each other. Message queues, you know, stuff like RabbitMQ or SQS, seem to be the go-to for keeping services separate and dealing with tasks that don't need an immediate response. But when do you *really* need to add one? Is it only for traffic surges, or are there other times it just makes perfect sense? I'd love to hear what you all think and when you've found them absolutely essential.