1. REPL. Huge. You can be insanely productive with it. If you're integrating with APIs and therefore dealing with complex nested data structs, you can explore them live. You can explore what's possible with any lib, try things before you write any code. You can also debug errors on the fly as soon as they occurred, right in the browser.
2. Your app is organized around entry points by default. Nobody knows the proper architecture up front for a brand new application. Rails gives you rake tasks for CLI, controller actions for web requests, ActiveJob as a convention for background jobs. You can grow your app's inner architecture over time, but you got all the entry points laid out up front, and they rarely ever change.
3. A lot of built-in security risk mitigation. Cross-site request forgery, encryption for cookies and sessions, convenient encrypted configs and secrets, log filtering functionality, escaping everything by default, and many more. Everything is already handled out of the box.
4. Veteran open source packages (gems) with large companies sponsoring maintenance. Decades of dev and polish on most needed gems, still going.
5. Analytics and monitoring out of the box. Completely wired for tracking performance of everything out of the box. No need to manually integrate performance trackers, just one-line install and you have full profiling capabilities of every part of the app, with many popular dashboard services.
6. Error monitoring out of the box. Many ways to notify exceptions via chats, email, and services, with typically zero integration work.
7. Completely thought-out testing environment out of the box, for both integration and isolated testing. Clean slate on every test without manual setup.
8. Lots of Ruby and Rails tools/functions (e.g. Enumerable) for transforming data via functional pipelines (select, reject, tally, map, take_while, etc) out of the box.
9. Ability to write incredibly reusable code to solve a problem for all data types, not just one particular type (this has gotten better in Go with generics).