Do you mean why is grist hydrated on the way into the mash tun (from the auger)? If so, it's because it's ALOT easier to hydrate as it drops into the mash tun than it is to mix dry grain with strike water by hand in the mash tun. I've worked on systems where we did not have a mill and we dumped pre-milled grain into the mash tun through the top manway and then added strike water and then mixed. Usually about 2 bags of grain at a time. It works fine.
But having now that I have a mill, auger and hydrator I definitely prefer this way. It's easier. It's also more consistent and I find it more reliable regarding liquor:grist ratio and mash temperature.
Either way works. But if you're going to have a mill and an auger then you might as well have a hydrator too.
Or are you referring to somthing else?
Brooklyn Brewery at the Culinary Institute of America
Hyde Park, NY