[KolenCh]

It eliminated some ambiguity. It should be quite self evident that even without an example it is quite impossible to eliminate all ambiguity (it’s a feature of human language.) The more important property is that it never introduces more ambiguity. Ie at worst it doesn’t help, but not making it worse.

[pianom4n]

It did introduce ambiguity though. Without the comma it is 100% clear that sentence lists 3 people.

[mikebenfield]

Practically that would be very likely to be the case to the reader, but grammatically no, it's still ambiguous.

> They went to Oregon with Betty, a maid and a cook.

Betty could be both a maid and a cook. So there is still ambiguity.