Why should calculating how much money to give the government cost money?
Because of the time it takes to record and distribute everything and pay the government.
Paying for something helps but doesn't entirely free you from a responsibility to understand this flow. Because an employee is going to ask you why they have X on their payroll and not Y like they calculated on their end and then you have to spend an afternoon reverse engineering everything the tool did because "our payroll tool just did it like that" isn't a very professional answer.