Posted By: 
warshawlawFirst, it avoids the issue of opinion disclaimers (you know, the "I think it was colored but I don't know" stuff).  Second, it reassures buyers that even though the card is altered it is genuine.  Third, it is a nice presentation for the card.  
I know I bought an E103 slabbed as authentic with some color in the background and felt it was a better addition to my collection than a raw one.