Even a relatively small income from a retirement job has the power to reduce the amount of money you need to retire..
You should have enough sufficient retirement savings that you will not have to withdraw more than 4% of your assets each year to live on. At that rate you should have enough money to last all, or at least most, of the way through your retirement without running out of money.
If you have $500,000 in savings you could pull out 4% a year, or $20,000. To reverse that logic, if you can reduce your withdrawals by $20,000 a year by getting $20,000 from some other source, you could get by on $500,000 less in savings.
This only adds up to $500,000 if you can bring in this supplemental income throughout your whole retirement. And that may not be realistic. But every year you can avoid taking a withdrawal definitely stretches whatever you do have saved up.
If you haven't saved enough for retirement, or you're worried whether it will last, looking for a little supplemental income is a great solution. Check out the Retirement Jobs section of this site for some ways to bring in that additional income.