So, when should you actually use an API Gateway, and when is it better to just have services talk to each other

We're setting up some new microservices and have been arguing about whether we really need an API Gateway. For outside clients, it just makes sense for handling things like auth, rate limiting, and routing. But when our own services talk to each other, is there actually a good reason to use the gateway? Or does it just slow things down and create a weak spot? Just wondering what everyone else does and what you think about when deciding this.