So, when do you actually need an API Gateway?

I've seen so much chatter about API Gateways recently, especially with microservices. It feels like everyone puts one in front of their services. But I'm wondering when it actually becomes a *must-have* instead of just something that's nice to have. For example, if you only have like two or three services, is it too much? When do the advantages of having all your routing, authentication, and rate limiting in one spot start to be worth the extra hassle? What do you all think, and what have you experienced?