It's true that a single senior with $21K of SS income only would generally not have a federal filing requirement.
There are some benefits to filing a return even when someone doesn't have to:
1. Refund of any withholding that may have occurred.
2. Help prevent tax identity theft.
3. Occasionally there are tax rebates that are only sent to tax filers. My state issued two of these last year, and there have been several at the federal level in the past several years.
4. It's possible, but unlikely, that the rules for who must file a tax return might change from year to year, so it might be worth checking to be sure.
There are a number of refundable tax credits which you can look at towards the lower part of the Form 1040 (around the area where withholding is listed) and on Part II of Schedule 3. It's highly unlikely that a senior would qualify for any of them, but not technically impossible.
Whether any of the above are enough to motivate a senior in the situation you describe to file or not is situation dependent.
AARP Foundation Tax Aide will generally prepare and e-file federal (and state, when applicable) returns for such people for free. They focus on low income and people over 50, but they'll do the work for anyone. Membership in AARP is not required and there is no "selling". The VITA and TCE programs do similar work.
