Arizona is not the only state that offers special number plates to raise money for charity, but it was one of the first. The desert state sells number plates supporting things as diverse as saving mountain lions to helping the homeless.
Though these number plates cost more, the extra funds aren't used as a tax raising mechanism but go to the good causes they promote. In contrast, the extra cost of private or vanity number plates do go into state coffers.
Having a number plate that promotes a good cause is a way to make a statement and personalise a road vehicle. Not all states offer number plates that support charities but most do already have private number plates.