Nah, it's a perfectly valid question.
The function of a water change is two-fold- to remove some portion of the bad things that accumulate in your tank, and to replenish some of the good things that get used up.
If all you were worried about is the first (removing things), then any salt will do. This is often the case with folks who have a FO (Fish Only) or FOWLR (Fish Only With Live Rock) tank.
If you have a reef tank with corals, both functions are important. Different salts will provide a balanced set of ingredients that help you maintain Calcium, Alkalinity, and Magnesium.
Another consideration is that some salts are more consistent from batch to batch. If you're testing your make-up water and adjusting its parameters to closely match your tank, then having a consistent amount to add each time is a boon.