The first thing ATMs (at least over here) do is check if you have enough balance for your transaction. If you just stored events, you'd need to collapse all the

The first thing ATMs (at least over here) do is check if you have enough balance for your transaction. If you just stored events, you'd need to collapse all the previous delta operations at that point, which would be arbitrarily slow if you never persisted data. Storing event records vs materializing changes is sort of an old hat trick in databases. People did that in the early 90s for better TPC-C scores. It has its uses, and in some contexts storing deltas can have huge advantages (e.g. bigtable with a log structured file system), but it's no silver bullet.