Because withholding may be too much.
Ifyou traditionally receive a large refund, re-think whether you want the government to have your money, interest free, for twele months. Additionally, consider that refunds have become less “seamless” and more often delayed than in the past.
Because withholding may be too little.
If you are receiving advanced Child Tax Credit in monthly installments ( a new process for 2021), that amount will be reduce the Child Tax Credit refundable on the next income tax return. Your refund may be reduced or your balance due may be increased, depending upon the amount of your withholding.
Because your life situation has changed.
If you have more or fewer dependents, or your marital status has changed due to marriage, divorce or death, or you have more or fewer part-time jobs this year compared to last year. Life changes usually change tax liabilities and warrant a review of your withholding.
Your employer can give you the form to adjust your withholding in order to do it yourself, or contact your tax preparer.