AFAIK the reason one does this is because one can (in most non-shitty databases cough cough) apply most or all transformations to schema inside a transaction, a

AFAIK the reason one does this is because one can (in most non-shitty databases cough cough) apply most or all transformations to schema inside a transaction, and also write the current version in that transaction, such that if the transaction completes the risk that the version you think the DB is on and the version it's actually on don't match is minimized, if not eliminated. You can't really get that unless you tie it to the transaction on the DB level, which means writing a version somewhere in the DB, in some fashion.