What does "trust proxy" actually do in express.js, and do I need to use it?

I am writing an express app that sits behind an nginx server. I was reading through express's documentation and it mentioned the 'trust proxy' setting. All it says is >trust proxy Enables reverse proxy support, disabled by default I read the little article here that explains Secure Sessions in Node with nginx. <http://blog.nikmartin.com/2013/07/secure-sessions-in-nodejs-with-nginx.html> So I am curious. Does setting 'trust proxy' to true only matter when using HTTPS? Currently my app is just HTTP between the client and nginx. If I set it to true now, are there any side-effects/repercussions I need to be aware of? Is there any point to setting it true now?