Answer:
The monarchs taxed some people because taxation has always been the main source of income for any political regime.
Monarchies were not the exception, and monarchs needed resources for keeping armies, paying some government officials, and for obtaining their own private wealth. These resources came mainly from three sources: earnigns from trade with other states, the printing of coins (seignorage), and taxes.
Taxes were the main source, and they could be charged in monetary values or physical goods.
