Commas and Coordinating Conjunctions Before Dependent Clauses

"For what do we live, but to make sport for our neighbors, and laugh at them in our turn?" -- Jane Austen

Many famous authors and writers use coordinating conjunctions and commas in this way. Are these commas used to make it easier to parse, or are they indicating a pause, or are they indicating a tonal shift? What are the specific guidelines for commas preceding a coordinating conjunction that is between an independent clause and subsequent dependent clauses?

In the example sentence, I understand why there is a comma between "neighbors" and "and": the coordinating element contrasts the inversion of the dependent clauses. I, however, do not understand the comma between "live" and "but." "For what do we live" is an independent clause, not an introductory clause.

Here is another example from a college writing center's website.

"When connecting two independent clauses with a comma and a word like 'however' or 'therefore,' you can either divide the sentence into two sentences, or use a semicolon, which can be used to connect two independent clauses without a coordinating conjunction."

So, what do you think?