The big one as a layman is they've invested in tooling for "edge" compute - similar to CloudFlare distributing your static assets worldwide for faster responses

The big one as a layman is they've invested in tooling for "edge" compute - similar to CloudFlare distributing your static assets worldwide for faster responses, Fly gives you the tools to do the same for your app servers. If you've got a worldwide audience you can set up your app in Dallas, Paris, Chennai, and Tokyo, have it auto-scale (and re-balance) instances based on which regions are the most in demand (eg, minimum 2 app servers per region, max 20 app servers, and it'll auto drift them throughout the day as needed). It also puts all your servers on a common VPN for communication, and they've got some tooling in place to help manage things like db replicas, replaying write requests back to the primary db env, etc. It's not necessarily trivial to port any old app over (if you're doing writes every request for logging etc, that's something you'd want to change up backgrounding the writes) but it really can make a huge impact in responsiveness once you're set up.