It allows you to query a system (some databases support this) in a temporal context. This is useful in financial services where you have market data baked into a price you've offered to a client, where the underlying market data might have been later corrected by a vendor.
So you can query how the world looked last tuesday 4pm; but also - given what we now know, how ought the world have looked last tuesday 4pm.
Interesting stuff.
A long time ago, I worked on a such a system that provided service-based billing for an industry where rates were very fluid. Having this "at any point in time" view of key data was essential for many things.
https://www.cockroachlabs.com/docs/stable/as-of-system-time....
I haven't used this feature, is it possible to twist this to find a time associated with a table state so that you can then query the table at that time (allowing you to answer a question like what was the value of X the last time Y was true)?
SELECT * FROM employees
AS OF TIMESTAMP
TO_TIMESTAMP('2004-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')
WHERE last_name = 'Chung';
So many LOB applications (the part of the iceberg under the water) have modestly sized databases with a relatively consistent number of users. The data is small, but often complex, awkwardly structured, highly relational, etc. The challenges these applications face are different, but real.
An example of something that would be incredibly useful here but completely untenable for a Facebook scale system would a rich rewind mechanism. You can go back in time 15 minutes or execute a query like:
RESTORE my_db TO LAST POINT WHERE EXISTS (SELECT * FROM ACCOUNTS WHERE ID=1234)
How much trouble would that kind of query have saved you in your career? At some point this would be cost prohibitive but I bet 50% or more of all apps never reach that point.