Most employees never write a cheque to the tax authority, because the money is removed before the salary arrives. The mechanism solves a collection problem and creates a perception one.
Collection risk is the original problem
A tax assessed annually and paid afterwards depends on taxpayers having set aside the money, and a substantial share would not have done so.
Deducting at source removes that dependency entirely. The employer calculates and remits the amount, and the taxpayer never has custody of it.
The arrangement also improves the timing of government revenue, converting a single annual receipt into a steady flow that matches ongoing expenditure.
The calculation is cumulative or periodic
Some systems calculate each pay period independently, applying annualised bands to that period's income, which is simple but handles variable pay poorly.
Others track income cumulatively across the year, so each deduction corrects for what has already been paid, and the total self-corrects as income changes.
Cumulative systems handle mid-year job changes and irregular bonuses far better, which is why an unusual payment can produce a strange deduction that reverses in later periods.
Accuracy depends on information the employer lacks
An employer knows what it pays but not what other income exists, what deductions apply, or what a spouse earns where joint assessment matters.
Withholding is therefore an estimate, adjusted by codes or allowances that the taxpayer or the authority supplies, and it is only as accurate as that information.
Second jobs are the most common source of error, because each employer applies allowances as though it were the only source of income.
Refunds are not a windfall
A refund means more was withheld than was due, so the taxpayer lent money to the authority at no interest for part of the year.
Because the money arrives as a lump sum, it is often experienced as a gain, and adjusting withholding to eliminate it feels like a loss despite improving the position.
The opposite case is worse. Under-withholding produces a bill later, sometimes with interest, and the amount can be difficult to find after the income has been spent.
Where withholding does not reach
Self-employment income, investment returns and rental profits usually fall outside payroll withholding, so those taxpayers must estimate and pay directly.
Systems typically require payments on account through the year, based on the prior year's liability, which reintroduces the saving discipline that withholding removed.
Because thresholds, payment dates and penalty rules differ by jurisdiction and change regularly, anyone with income outside payroll should confirm current obligations rather than relying on past practice.