If I get a lot of mileage out of my car, it means I use my car a lot. If I get a lot of my mileage out of my microwave, it means I use it a lot.
If I'm a writer and I get a lot of mileage out of my childhood experiences, it means I use them a lot, in other words I write about them a lot. It tends to be a little derogatory, in the sense that as the writer, I overuse refe
Yes, the idea is to gain maximum advantage from whatever. Giles and the Jews are not simply talking about those terrible events, but are using them as defense, rationale, corroboration, motive, and so forth (I quote the opinion of the writer, of course).
A more common and slightly crude phrase with the same meaning is 'to milk it'. When one milks a cow, one assumedly keeps milking it until one has acquired as much milk as one thinks possible. The same can apply to anything you wish. Similarly, the phrase 'to milk it dry' emphasises that the object has been used to excess, and (not always literally) cannot be used any longer.