Well, as it turns out, thanks to the software-eating-
the-world thing and the Internet of Things we happen to be
amassing *vast quantities* of all sorts of data [...] The
demand for systems that are capable of storing and
retrieving temporal data on an *ever increasing scale*
necessitates systems that are specifically designed for
this purpose.
Strongly implying the big deal is the need to scale?"Given a value of the database, one can obtain another value of the database as-of, or since, a point in time in the past, or both (creating a windowed view of activity). These database values can be queried ordinarily, without having to make special queries parameterized by time." http://www.datomic.com/rationale.html
So in other words you get to see exactly how the whole database looked at any given moment through history (certain parallels with the blockchain I suppose).
Datomic is an ok choice in some contexts, but the one detailed here is not one of them, you're likely to reach Datomic limits quickly and be in a world of hurt when you do.
This is known as "time travel queries". Postgres supported this back in the original version from the 1980s but then they took it out in 1997 because it takes up too much disk space.
It's trivial to do in an MVCC system. You just turn off garbage collection (vacuuming in PSQL parlance).
I only noticed it because I was shocked that there was an actual comprehensible value proposition in a blogpost about a NoSQL product.
But yeah I guess if you really need NoSQL this should be nice. However sharding based on time will probably be one of the easiest approach for horizontal PostgreSQL deployment.