Some things I thought of: governmental registries, tracking history of physical assets (e.g. tracking of maintenance history of aerospace components), software licensing, storing timestamped cargo manifests (like the Maersk case). You could even build a currency on this with the right set of privileges.
That said, this is obviously not meant to store large amounts of data. But you could quite easily build a currency on top of this (add a 'balances' table, and use grants to determine who can update what - a future version could even provide smart contracts through SQL stored procedures).
Quite the opposite. Using blockchain for anything but timestamping is
worthless, because there are faster and less resource-intensive protocols for
about everything. Data storage, for instance, has distributed hash table, and
change coordination has leader election or consensus protocols.
> Bitcoin, Ethereum and others don't use a blockchain just for timestamping either, but store all sorts of data in it (transactions in Bitcoin and even complete programs in Ethereum).
You're putting the cart before the horse. These "all sorts of data" in
blockchain are in fact documents that get timestamped -- the timestamp
says "document X was issued before document Y". A Bitcoin transaction in
isolation from all the other transactions is nothing more than a semi-random
string of bytes. For it to be a money-like thing you need to have previous
transactions (and how do you know they're previous? by timestamp) that
testify the account had enough resources for all the output.
The idea that some of these transactions store data is not any different from the idea that some transactions in Bitcoin add new coins to the system, or that some transactions in Ethereum create and store contracts (which are code and data).
Now of course you could, instead of storing the actual data and/or SQL, put data in a DHT and reference and timestamp that.
And this should be the very first thing to put in README. This is the single
most important thing about any program that uses a blockchain protocol: what
exactly is the decentralized, adversity-proof timestamping used for. Right
after that should be placed rationale, i.e. how does the hostility resistance
help compared to cooperation-based protocols.
What "it"? Your question does not match anything I said.