when should i consider a message queue?

So, I'm noticing a lot more systems using message queues, you know, like Kafka or RabbitMQ. I kind of get the whole idea of separating services, but I'm having a hard time figuring out exactly when it's obviously better than just calling an API directly or doing simpler asynchronous stuff. Are there particular situations or problems that make you instantly think, 'Okay, this is where a message queue makes sense'?