There's been problems reported of people that paid for parking passes but couldn't get into their lot when they got there because the parking attendants were accepting cash from people without passes to park in reserved lots. With the scanners they can track exactly how many cars with legitimate passes get into an assigned lot and will know when they get reports from irate fans which attendants are pocketing cash or at least can gather info to help figure out the problem. When they were punching the passes they really couldn't verify anything.
I reported an attendant last year in the public lot I park in. The university knows how many passes they give each attendant, so when they tally their take, they count passes given to them minus passes returned, multiply by $20 and know how many cars theoretically parked in that lot. This guy would point blank tell you it was $20 if he gave you a pass for your window or $10 if he didn't. He would take your $10 and put it in his pocket and the university doesn't know the difference because they aren't patrolling looking for passes in windows. He was making $10 for each car he did that to and university was losing $20.