From https://news.ycombinator.com/item?id=14724841
Cite
I might be somewhat biased on this, but I strongly prefer database metaphor over computing metaphor.
Every blockchain is essentially a shared database. E.g. Bitcoin is a database which keeps track of address balances (as a first approximation) and enforces particular authorization rules. It’s a fixed-functionality blockchain.
On the other hand, Ethereum can serve as an arbitrary, user-defined database. Ethereum smart contracts are essentially like stored procedures in SQL databases — they can be used to enforce arbitrary consistency rules.
And the reason why transactions are quite expensive is that you’re paying to update everyone’s database. You can modify things locally for free, but if you need everyone to see your information, you gotta pay for it.
And computation is there just for the sake of enforcing arbitrary constraints.