I think Datomic which has been doing time series data for years now answers the "what’s the big deal" question for me:

"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).