This is an enlightening distinction that Gordon MacDonald makes in Ordering Your Private World:
In my earliest years of ministry, when this business of mental growth had not yet become a discipline for me, most of my study was what I now call defensive study. By that I mean that I studied frantically simply because I had an upcoming sermon go preach or talk to give. And all my study was centered on the completion of that task.
But later I discovered the importance of something I now call offensive study. This is study that has as its objective the gathering of large clusters of information and insight out of which future sermons and talks, books, and articles may grow. In the former kind of study, one is restricted to one chosen subject. In the latter, one is exploring, turning up truth and understanding from scores of sources. Both forms of study, offensive and defensive, are necessary in my life.
We grow when we pursue the discipline of offensive study.