Commercial two part solutions (like ESV B-ionic) claim to add all important elements so that using the product is effectively adding just calcium and alkalinity and since the salinity rises a bit, doing a small water change. The Balling method claims that as well. Magnesium should be stable with such a recipe.
So if the tank is using trace elements (things like iron, copper, manganese, etc.) it may be desirable to supplement them if they are not coming in from water changes and foods and such in adequate amounts.
Elements such as potassium and magnesium are not trace elements (they are called major ions), but a good commercial two part will contain them and may help keep them stable (certainly magnesium; potassium depends on how it is being depleted, if at all).
The two part from BRS (which uses my DIY recipe) is a different category of product. It certainly adds many minor and trace elements, but mostly as impurities and perhaps not in the quantities that a commercial two part would. So the need for them may be higher. The only way to know for a given tank is to either test for them (such as with Triton testing), or supplement and look for a benefit (or not).
FWIW, the BRS two part should maintain calcium, magnesium, alkalinity, chloride, and sulfate at appropriate levels when used as I recommend. Anything else is just a contribution from impurities, although my original recipe using Dowflake added close to the exact right amount of potassium as well since it is present in the Dowflake at a good level.
Any deficiencies or excesses in trace elements may be partly offset by water changes, and I recommend them for that reason.
