If you're a developer that uses a VPN (either on your computer or your phone), it can be frustrating accessing your "localhost" on your phone to test your work.  That's where services like ngrok come in so handy.  

They basically create a secure tunnel to a specific port on your machine.  Then, if the website/app you're developing is on localhost:3000, you can access it via a unique URL like https://abc123.ngrok.com from any device.  

You can also let others access your local computer from wherever they are.  You could also let your client or coworkers give you feedback/advice as you develop locally without having to publish the project.

In this video, I'm going to show you how to use this functionality while developing React Native apps using Expo.  You can access your local development work from any other device in the world thanks to a built-in tunneling feature.