It is a reference to Don Quixote attacking windmills because they were giants oppressing the people. Which, today, just seems ridiculous.
However the main reason to switch from hand mills to windmills was that it was far easier for the miller to collect taxes when the grain was milled than for tax collectors to go to every peasant and collect taxes there. Not only was it more efficient, but lords who forced their peasants to switch often raised taxes simply because it was easy to do so.
This was all current events when Cervantes wrote Don Quixote. And therefore his audience would have been expected to understand that windmills truly WERE "giants oppressing the people"!
No point. Just fun trivia about how different a modern phrase looks when looked at from the point of view of its history.