The "shared ledger" analogy for blockchains is limiting. If you want to understand the potential applications, you should stop using it in your thinking. Blockchains give us publicly writable databases . Such a thing never existed before. A blockchain has one or more programs that define what can be written to the public database. Bitcoin's program is mostly about distributing the 21 million coins to miners and preve…
It gives you a publicly writable database that is only secured by people desiring to throw away large sums of computation in an environmentally unfriendly manner. Writes are also slow (10 mins...) and requires relatively high bandwidth and space to be used securely. There's no reason that AirBnb or Uber would want their database to be public, slower, and broken as soon as people lose interest in splitting up mining.…
Blockchains are harder to break than most corporate databases.
Reading up on Ethereum will answer most of your questions.