For when your corals grow faster than the trace elements in your was. In that small of a tank you will never need to use those unless you have tons of acropora/sps which I don't reccomend for a beginner/such a small tank.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"