It's useful in some scenarios, but I think the biggest benefit is debugging (something weird happened in prod? what was the database state?).

I think people try to stretch its usefulness to scenarios where it doesn't quite fit. It almost fits for historical reports, but from my experience, usually you're going to find an error in your data that you want to fix "in the past" so those old reports change, and all of a sudden datomic doesn't automagically work for your historical reports anymore.

It means that when you figure out how to calculate business metrics based off of information stored in Datomic, you get to compare how your business was doing versus how it's doing now for free. Or if you figure out a customer health score, it's now trivially easy to calculate how much it changed since last month.

I used it to show the edit history of forum posts until I found out the hard way that you're not to store long strings in Datomic.

Let's say you are creating Git like system so you will need some data at specific time right?

Auditing, which might even be a legal requirement in some application domains.