Usually to treat or prevent hypokalemia (low potassium) Potassium is an electrolyte and helps the body function properly. Some drugs like diuretics remove potassium from the body so potassium supplements are given to prevent the loss of too much potassium.
Potassium is used for several functions in the body especially the beating of your heart. As DzooBaby already explained a condition called hypokalemia can develop because of some medications or the loss of fluids such as with diarrhea or excessive perspiration which would then require potassium supplementation.
Hope this helps,
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