I couldn't understand when I should consider to use Datomic Ions.
What are the benefits besides having plain Clojure + Datomic project setup?
The benefits are listed at https://docs.datomic.com/cloud/ions/ions.html#benefits. Develop your functions at the REPL, deliver them on AWS a minute later.
For an extended walkthrough, check out https://www.youtube.com/watch?v=3BRO-Xb32Ic.