Scanning means checking for errors and/or failures (parity inner errors/failures and parity outer failures, PIE, PIF, and POF). Some drives (not all) are capable of checking these. However, not all are created equal in their abiltiy to accurately scan these. Lite-Ons and Benqs seem to be the most consistent with this feature.
Some people scan just to post their exceptional scans, others to check consistentcy among media, and it can also alert you to problems with a burner before it starts producing an onslaught of coasters. It's up to you to decide how anal retentive you want to be with this if you even do it at all. (I do every disc, so that if I have problems later on with a disc, I can go back and rescan and compare).
You can use Nero CD/DVD speed to scan. This is the place to go cdspeed2000.com. Easy to use, and several tests you can run.