Why is “a” used here: “When a Mr. Andrews, the butler...”?

It is pretty straightforward. The indefinite article here is used here to indicate that Mr. Andrews is not well known. If you said "Mr. Andrews called..." it would indicate that you and the hearer both knew who Mr. Andrews was. If you said "A Mr. Andrews called..." it would indicate that you did not know who Mr. Andrews was, all you know is that his name is "Mr. Andrews".

It's not necessary but is used here to emphasize that you don't know the person and his name isn't important - he could be any Mr Andrews.

Similarly it's used insultingly to de-emphsize Napoleon. "when a defeated Napoleon rode off the battle", meaning that this was somebody unimportant who happened to be called Napoleon!