Yes it was for auto-reviewing new revisions. New revisions are seen as a combination of (base revision, changes). If the base revision was reviewed and the user is trusted, then so is the new revision. MW core had the obvious cases of rollback and null edits, which are (base revision, no changes). Their is a lot more "base revision" detection in FlaggedRevs for the remaining cases, some less obvious (user supplied baseRevId, X-top edit undo, fall back to prior edit).
If baseRevId is always set to the revision the user started from it would cause problems for that extension for the cases where it was previously false.
It would indeed be useful to have a casRevId value that was the current revision at the time of editing just for CAS style conflict detection.
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