Because guaranteed transactions matter. Moving $10 from Bob's account to Jack's involves: 1. Inserting a transaction to remove $10 from Bob's account 2. In

Because guaranteed transactions matter. Moving $10 from Bob's account to Jack's involves: 1. Inserting a transaction to remove $10 from Bob's account 2. Inserting a transaction to add $10 to Jack's account. Both 1 and 2 must succeed or fail in total if either of them fail (or both). You can imagine the disaster in banking apps, ERPs and other financial systems when 1 succeeds but 2 fails and the database is happy with that imbalanced state.