@Terence already gave several great answers to this question, but I will add my own answer. Trident is able to track Mg with such precision that in some ways, it is able to function as a surrogate for monitoring salinity. I've seen a couple of different situations on a couple of different tanks where there was a problem that was affecting salinity, and the first indication of the problem was a change in how Mg was tracking. Even if a tank has a salinity probe already, having the Mg number track together with the salinity probe's readings provides extra confirmation of what the salinity probe is showing.