Why was a cooling system additive used when car was serviced? Car Doctor

I had the radiator serviced on my car; just preventive maintenance. I noticed on the repair invoice I was charged for a cooling system additive.
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7 reasons your check engine light comes back on after being reset

A check engine light that returns after being reset means the underlying fault has not been repaired, only cleared from the vehicle’s diagnostic memory. The OBD-II system monitors engine and emissions ...