when should you think about using message queues for microservices tho

so, i've been messing around with microservices a lot lately, and message queues are always coming up. we have services that need to chat async, and sometimes sync. i'm trying to nail down when it's actually better to use a message queue, like Kafka or RabbitMQ, instead of just calling services directly, even for async stuff. what have you guys found? what do you consider when deciding? is it just about decoupling, or do speed things matter too?