The amount of calcim required for a high demading tank..
If you add supplmenets, espically to a large tank, you will be adding it constantly to keep up with the demand of the tank. With a reactor, it already does it for you.
Not to mention, over a 3-4 year period, the reactor will have paid for itself over the use of the supplements.
When you go on vacation, do you trust someone to dose the tank right?
Just a couple of advantages.
The cost of the reactor and all that goes with it is the disadvantage.
There are more scietific reasons why they are better but I belive you just wanted some general..