If you are referring to coins graded at PO1 which is the lowest grade a coin can be given, that is because they are in known as lowball coins. There are collectors who collect lowballs and the lower the grade, the more it is worth. Many times it is harder to find a low-ball than a high grade in the same series. As far as "roughing it up" to make it look worn, it is usually easy to tell between a genuine low-ball and a manufactured one. To be a genuine low-ball, you have to be able to tell the date and denomination, a coin that is worn so much that it looks like a blank has no value.