Digressing from Negative Overload training and going back to the eccentric portion of a normal rep... I think a lot of beginners focus more on what they can lift and how quickly (insert ego here), without realising the benefits the eccentric potion of the rep brings. Nor do they understand, or are rarely taught, how a controlled eccentric motion can contribute to muscle growth. And building on what CCM said, a slow, controlled eccentric ensures an even stretch across the entire muscle and it's associated ligaments and tendons, thus contributing to improved ROM in the long run.