> One way of thinking about a blockchain is to think of it as a shared datastructure to keep databases in sync. Any time you want to distribute your database over more than just a single central place, in a cryptographically secure way, you're probably going to re-invent a blockchain to do it.
Even more specifically, a blockchain is for when you want Byzantine fault tolerance, i.e. you don't trust one or more of the actors involved. This is the main distinguishing feature of blockchains IMO, the reason we have proof of work, proof of stake, etc. It's also the main thing I saw people getting wrong when using blockchains during the earlier waves of cryptocurrency fever; most proposals for blockchains did make sense as distributed public ledgers, but didn't really need the extra computational overhead because only trusted parties were adding blocks to begin with.
Even more specifically, a blockchain is for when you want Byzantine fault tolerance, i.e. you don't trust one or more of the actors involved. This is the main distinguishing feature of blockchains IMO, the reason we have proof of work, proof of stake, etc. It's also the main thing I saw people getting wrong when using blockchains during the earlier waves of cryptocurrency fever; most proposals for blockchains did make sense as distributed public ledgers, but didn't really need the extra computational overhead because only trusted parties were adding blocks to begin with.