All reactors are available to make your life a little easier, if setup right a CaRx will keep your levels where you require it.
CaRx really come in to play when you have large amounts of stony corals and manual dosing becomes a problem or "too lazy"
"Calcium carbonate reactors have become a popular way of replacing the calcium and carbonate taken up by corals in the process of calcification. In its most basic form, a calcium reactor is simply a container filled with calcium carbonate (CaCO[SIZE=-1]3[/SIZE]) media over which aquarium water is passed with the addition of carbon dioxide. Adding carbon dioxide lowers the pH of the water, making it acidic, and dissolving the calcium carbonate to provide the aquarium with calcium and alkalinity."
Agreed. Due to the decrease in PH most people will either add a Kalk reactor or dose manually as this increases PH. Must say until you have a good amount of corals that will consume calcium and bicarbonate a good salt used with regular water changes should maintain your levels.
It is expensive pieces of equipment. Calcium reactor + CO2 bottle and regulator + PH probe + PH controller.