It's about untrusted developers. Let's say you've got source code control. (You do have source code control, right?) No matter how badly someone goofs up, it's relatively painless to roll back to the last known good state.
So what procedures do you have for rolling back the DB? BEGIN TRANSACTION ... ROLLBACK is nice when you know before the procedure is done that it was wrong, but once you COMMIT, then what? It only takes one mal-formed (or missing [shudder]) WHERE clause to destroy a day's worth of data. So how do you protect the data from bad DELETE queries?