My one sentence take is that blockchains allow the abstraction of trust.
In an example like Reddit, the points are centralized in their database and trust is implicit that they are tallying them correctly. There is not much use to putting that information on chain because its slower and more expensive than just a relational db.
In a system that would require you may need to trust someone you don’t know, like swapping an ASA for Algos where you don’t know the current owner, a blockchain allows you to trust that the transaction will proceed as expected. This does require you to trust the smart contracts and blockchain code do what is expected but both are auditable.
A good overview document is available here to reference