When you start to do withdrawals from your employer sponsored retirement plan (401k or 403b plan) the IRS requires a 20% withholding on the distribution.   That 20% is applied to your taxes and if you are in a lower bracket on April 15th you'll likely get money back.   That 20% rule is mandatory and some states will also have a withholding.  Many retirees are surprised at the tax bill as they start to take money from their plans.

If you are retired or going through a job change it might make sense to roll funds from the employer plan into your own IRA, especially if you need income or anticipate withdrawals.  The rollover is not a taxable event and distributions from the IRA will still be taxable but the 20% rule doesn't apply.  You can elect any percentage 10%, 15% etc or elect not to have any taxes withheld.

If you decide to keep money in the employer plan, your MRD at 70 !/2 is not subject to the 20% rule.  The distribution is taxable but you're not required to do the withholding.