Even if you are retired or only get SS income if you have self employment profit you still pay the SS & Medicare tax. And it could potentially increase your SS benefit.
Working after getting SS benefits. Your SS can increase. See 4th paragraph,
We check additional earnings each year you work while receiving Social Security. If an increase is due, we send a notice and pay a one-time check for the increase and your continuing payment will be higher.
https://blog.ssa.gov/three-common-ways-your-social-security-payment-can-grow-after-retirement
And growing your SS benefits after retirement
https://blog.ssa.gov/how-you-can-grow-your-social-security-benefits-beyond-retirement-age
