they do a couple things. one, you can only mix so much kalk with water, 1.5 to 2 table spoons per gal. the reactor lets you fill 4 to 8 oz in it and it will mix or keep the water mix with kalk at all times. it will dose you tank by pumping fresh water though it.
but because as you need to montior your ph while dosing.
here is a couple links where we have talked about it.
I like my Kalk reactor, it is a balanced additive, meaning it supplements Ca and Alk. In addition, it has a high pH (around 12) so it works well to balance my effluent coming from the Ca reactor. I think they are worth the investment, especially for a high quality one like a GEO, or My Reef, or any of the other reputable brands.
As stated above, they are great pieces of equipment, but VERY simple to make. However, a reactor is certainly not needed to get all the benefits kalkwasser has to offer. As someone mentioned above, you can mix it directly into your top off reservoir, then allow your ATO to dose it when your tank tops off.
One thing to remember... Kalk in a reactor needs to be stirred. Kalk mixed into a top off reservoir (still reservoir) does not.