ELI5: A blockchain is an »I told you so« store. Future statements/promises are linked to past ones so you can’t mess one without having to mess with all the others. It’s like a black board you can’t wipe clean again, adding more text (and space to write to) is the only thing possible.
For example, you could promise to paint your house red in a blockchain. You later regret that promise and tell your friends you never said that, but since they all have access to all promises made, they have good evidence that what you say is not true.
Thanks, very good explanation. So blockchain enables us to move from "our data warehouse is the single source of truth" to "our data warehouse is the single History of truth".
A simple counter-example comes to mind: rewriting history is trivial only if you want no central authority.
Even if we lose the "decentralized consensus" aspect with no PoW, it still can be advantageous. E.g.: simply having an authoritative server always responding with the latest block hash. The server is very lightweight (as in 80s microcontroller lightweight) and it can offload the actual blockchain handling to clients.
For example, you could promise to paint your house red in a blockchain. You later regret that promise and tell your friends you never said that, but since they all have access to all promises made, they have good evidence that what you say is not true.