I always wonder how much of that 400a actually gets used. What are these people's electric bills? Average 60a 24/7 is over $1000 a month here.
Generally speaking, in many places 400A service is the next step up if the load calc exceeds 200A. And, of course, the load calc is interested in making sure that the probable sustained peaks are covered; it's not about the average draw. So one might need a 400A service when the current draw averaged over the course of a month is only a few dozen amps or less.