Once I heard someone saying they were our response to the American fast-food. It could definetly be true, but the variety makes those kilo restaurants much more attractive than McDonald's.
It works like this: you receive a blank bill when you enter. You walk through this huge buffet with salads, pasta, harty food and all kids of meat. You get a plate and chose what you want to eat. Then walk to the clerk who weights your food and writes down how much you should pay.
And that's it. Why choosing for pasta or meat if you can have it all at once?
And it's so cheap you won't believe your eyes. Just for your information, a very hungry man eats about 700g of food in a meal. In such restaurants, you pay maximum U$1 per 100g. So, won't you try it?