I have never seen a recipe that called to potassium without also listing which of its forms was needed ... I assume you are talking about the CopyCat KFC Marinade
recipe that calls for 2 tablespoons potassium chloride
If so - it appears that in this case the marinade also acts as a curing solution ... and the potassium chloride is acting as both a flavor enhancer and an aid in moisture retention ... similar to a brine but not as salty.
Like Aurora said - it's often used as a salt substitute by people on sodium restricted diets.