It's a source of potassium (the K in N,P,K) - all plants need it for healthy growth, but it's normally recommended for flowering plants in particular, which obviously includes beans.
Light soils are more likely to be deficient than heavier ones, because potassium salts are highly soluble in water and easily leached away.
Most general purpose fertilisers, liquid and solid, contain it, and are probably sufficient for most plants on their own, but a supplement of potassium sulphate won't do any harm, although I'd probably stick to one dose rather than two, as you can have too much of even a good thing.
