Tarafdars were given lands "Tarafs" to manage. The land management included collecting taxes, both normal and agricultural taxes (Lagaan). A Tarafdar maintained all the accounts and handled all the monetary/financial transactions for his land.
The title was a prestigious position, so descendants continued claiming it, and it turned into a surname.
In Bengali, Tarafdars were known as "Khajanchi".
Tarafdar is a surname in Iran and in Turkey. In Persian, "Taraf" means "direction" and "dar" means "have" literally "to have a directin". Actually the combination means "in favor of"; e.g. "Tarafda:r e sb" means "In favor of sb". In Turkish it indicates someone who is a joiner of organizations.