It's useful to look back through the change history, read original design documents, and understand why a component was built the way it was. But you also need to look closely at how the component integrates into the system today, where it can easily have taken on additional roles.

Chesterton's Fence, the idea that if you want to change something—removing an apparently useless fence—you should first determine why it was set up that way:

In addition to considering why something was created, you also need to consider what additional purposes it may have since come to serve.