What is it you're studying?
In my experience they are, a lot of the articles I'd been assigned as essential reading expand on and help to explain a lot of the topics that are briefly touched on in lectures. Usually lectures are used as an overview to briefly mention the key topics and it's expected of you to go away and read and actually understand these concepts. There had been times where I'd skipped out on essential reading and had really not understood something as well as I should have done.
If you're studying an essay based topic, I really would just do the reading early on and as it's assigned to you since it's really easy to fall behind once you've got several essays to write. You'll be wasting time still reading up on the basic topics that you should be seeking to deepen by this point.