Nothing to do with Billy Ball or anything like it. This is not evaluating players, it is evaluating the result of all teams talent evaluation/drafting based on a look at history.
Everybody knows you should get a stud with a top ten pick and if you don't you blew it.
Everybody knows that a later first round pick should be a very good player, and it hurts when you miss.
But when you get into the second round and third round and later rounds, everybody can name the brilliant selection where you got what in 20/20 hindsight should have been a 1st rounder. Those guys are easy to remember because every commentator mentions it when they talk about those guys - heck they now call pick 199 in the draft 'The Brady Pick'. BUT what no one keeps track of in the media or in the fan community is all those other players that excited or depressed you when they were selected in round x because they were such great value or such a ridiculous stretch by the GM. You remember them for a year, maybe two, but they fade from memory, maybe not for the picks made by your team, but for the other 31 picks made in each round by other teams.
So looking at the metrics compiled by number crunchers is a way to evaluate how your team actually did compared to other teams. And to maybe temper your anger or elation over some pick in the next draft.
Why get ompletely bent out of shape when your team 'flushes' away a mid fourth rounder on a no-namer they could have picked up as a free agent when the reality is the player you wanted them to pick was odds on to fail anyway.
Think of this - If the Pats had used pick #91 on Da'rick Rogers all the fans and the pundits would be saying what good value they got at that point in the draft. BUT the reality is if they had done that, they would have been the only team of 32 that had a value greater than #255 assigned to him.