Probably because adding something to the kibble, I think, makes it smell and taste more appealing. Actually mine will eat their dry food, if I put a handful ont he floor last thing. Normally I add a small amount of tinned meat to the complete, more because I think it LOOKS more appealing than just a bowl-ful of dry kibble!! At one point I used to add some warm (NEVER HOT AS IT CAN DESTROY THE VITAMINS IN THE COMPLETE FOOD) water just before feeding, but these days I don't. One of the tinned brands I use has a lot of gravy which adds liquid just was well. I have also added some warm water to the kibble I use, to see how much it expands - not at all. It's a fact that if it expands a lot in the stomach, it could lead to bloat. Which is why I check when I use a new food.
ps Once or twice a week I add a cooked egg to their food.