Anyone who really wants to understand the Reformation deeply needs to get to know the patristic and medieval theologies that preceded them. Think of it this way, if you want to understand your parents, it's very helpful to know your grandparents and even your great grandparents. We're shaped by those who preceded us, who formed and shaped us. So it is in the church. Luther was a trained theologian in one of the prominence schools of late medieval theology. He was essentially a medieval (not a "modern"!) man. Calvin was influenced by a variety of medieval and Renaissance writers. That's true for Bucer, Bullinger, and others.
The Reformation was not a wholesale rejection of everything the medieval theologians taught but it was a significant reorganization of received ideas and doctrines. In that sense "Reformation" is distinct from revolution. The Anabaptists (which is arguably the greatest influence on American evangelicalism since the 1820s) has a revolutionary spirit. It isn't Protestant and it isn't connected to the broader Christian tradition (though it retained the worst of medieval moralism and mysticism). The Protestants, by contrast, criticized the Fathers and the medievals by using Scripture as a measure. Nevertheless, much of our theological vocabulary comes the Fathers and medievals.
Finally, it's important to know the medieval theologies, pieties, and practices in order to keep from returning to them unwittingly. In the modern period many Reformed folk have resurrected medieval worship (on the same principles that the medievals did what they did) and a variety of movements (e.g., the NPP and FV) have attempted to take us back to the predominant medieval doctrine of justification by grace and cooperation with grace.
We've been susceptible to all this because we've not always recognized the uncritical reintroduction of medieval ideas when it has happened right before our eyes.