So, when should you use a message queue instead of just calling an API directly?

hey everyone, i'm trying to nail down the decision-making process for when to introduce a message queue into a system versus just having services communicate directly via REST APIs. i've seen queues used for decoupling and handling load spikes, but also seen them add complexity where it wasn't strictly needed. what are your go-to indicators or rules of thumb for choosing between these two communication patterns