The main duty of the County Treasurer is to collect taxes, and then to turn these taxes over to the proper organization, position, or person. They also maintain and keep the county tax records.
The County Treasurer must be of utmost trustworthiness and reputability in their county. They handle almost all money that the county deals with, at some point. Therefore, graft can become common, especially among more unscrupulous individuals. They also must have in-depth knowledge of local tax codes.
The purpose of taxes is so the government can afford initiatives that benefit the public community. The County Treasure has first access to this money, and could misuse the funds, defeating the purpose of taxation in the first place.