Here is a good reason: [summary of this post which Andy mentioned]
During the period where browsers support both the vendor prefixes and the actual property - there might be differences in the implementation of the css rule.
Example:
.not-a-square {
/* These do totally different things */
border-radius: 30px 10px;
-webkit-border-radius: 30px 10px;
}
The spec or "real" version will render the top left and bottom right
corners at 30px and the top right and bottom left corners at 10px. The
vendor prefix will render all four corners with elliptical corners
30px wide and 10px tall.