Salt in coffee is supposed to mask the bitter substances and make the drink taste milder. The principle behind this is simple: sodium chloride, i.e.
table salt, influences the taste receptors of the tongue and masks the bitter note in the coffee. Theoretically, you should be able to add the salt even after cooking and achieve the same effect. However, it is better to add it to the coffee powder before cooking, as the salt crystals have more time to dissolve. Too much salt is not good either: The upper limit of salt is six grams.