A Delta table is Parquet files plus an ordered log
On the canvas
Two Parquet files at the bottom. Above them, commits v0 and v1 in _delta_log/.
In plain English
Every write appends one numbered JSON file to _delta_log/ — 00000000000000000001.json. Each commit lists add and remove actions. Version N is nothing more than the result of replaying commits 0 through N.
Without this step
Without the log the directory is the state. A half-finished write is immediately visible to readers, and there is no earlier version you could even name.
So what does an UPDATE put into commit v2?
