He wanted to understand the entire process of growing tobacco, as well as understand the entire experience had by those who farmed tobacco. Somebody who merely watched, would only understand what they could see. If you're a part of the process, you get to see the deeper meaning.
He couldn't say he truly understood such processes as priming tobacco (pulling off the leaves prior to drying) if he hadn't actually done it, even though the process looks obvious to the casual observer.
Student or Learner