Some diuretics are potassium sparing ones, which means when abused on an already depleted state hyperkalemia can hit you badly... very high blood potassium levels vs almost zero sodium... not good for your heart (severe risk of cardiac arrest)
Insulin, on the other hand, does the exact same thing with potassium that it does with blood glucose... lowers it.
So when too much k+ has been administered or when k+ sparing diuretics have been used in excess (or both), a shot of fast acting slin can quickly normalize your levels, thus avoid possible risks of death.