When should you add a message queue to your architecture?

Hey folks, i've been seeing more systems incorporate message queues lately, and it got me thinking about the tipping point. We've got a few services that could probably benefit from decoupling, but i'm hesitant to add complexity just for the sake of it. When did you first decide to introduce a message queue? What specific problems were you trying to solve, and were there any alternatives you considered before making the jump