Every DJ knows that you can see the loud and quiet bits of a record, especially on 12" singles where the grooves are nicely spread out. The loud bits look white when viewed under a lamp at the right angle, and the quiet bits look dark. This means when mixing, you can pick up visual clues as to when the breakdown is coming, letting you know how long you've got to complete a mix even if you don't know the record too well.

You can even see the individual beats on house records with a strong kick drum, especially if those on adjaecent grooves line up with each other. This gives a nice spiral pattern towards the middle of the record.