^It should be the case that peers in disciplines have a better take on what’s happening in their disciplines than, say, provosts would know about other universities as a whole. Department heads, graduate advisers, and leading faculty are often keenly aware of who is gaining or losing ground in their disciplines. I do not believe that any ranking is exact, but I publish the departmental rankings (and compare public and private university departments) because parents and prospective students should see the data in the most usable format, i.e., comparative.
I also think that both overall university academic reputation and peer assessments of disciplines are important to honors students (which I write about, and for) because even if a highly-rated discipline is in a large public research university, honors students can avoid many of the negatives of such universities by taking advantage of smaller honors class sizes and research/mentoring opportunities.