Data verification is only part of the picture. A failed verification does not mean there's any problem with the disc, or the data. Due to the way that many programs perform the verify, any number of hardware glitches can cause a failure.
Likewise, a successful verification does not mean the disc is "good". It can still have loads of problems that can result in a failure down the road, (with loss of data), failure in certain reading drives and not others, difficult or slow reading.
Scanning for read errors is a completely different process, and measures some sort of burn quality and read-ability virtues of the disc (in the testing drive).
Generally, a successful verify along with a perfect read-speed test will assure you of a reasonably good disc. But, if an error scan can give you a more detailed picture of the over-all quality of the disc, why not use that information to your advantage?