The purchase card program is fairly new in the federal government. I know in my former department/agency, there was quite a learning curve in using the cards. The cards were handed out without much oversight at first, then as problems arose, more oversight was initiated as the years went by. I think there were many more card users during first few years. Now, I believe there are fewer users with better oversight and more audits. My department/agency has a purchase card coordinator responsible for training and local audits and also national audits teams that also periodically do audits.
The purchase card program initially was a means of reducing costs associated with cutting purchase orders for every purchase. Purchase orders and employees needed for creating amendments, receiving reports, fiscal accounting, payments and late payments, and late payment fees all were streamlined by using the cards.
Plus the government gets that rebate check for using them.