when should you use a message queue, you ask?

so my system is getting a little complicated and i'm wondering when's the best time to throw in a message queue like Kafka or RabbitMQ. we've got a couple of services that need to chat without needing an instant reply, and it's not the end of the world if a message takes a little longer to get there. what kind of things make you decide to add a message queue to your setup. what are the first hints that it's a good idea