"He has a "B" on his back for Back Judge, but he's positioned as a Line Judge" says random fan, while looking at the referee Wikipedia page during a Cincinnati Bengals practice. I haven't the faintest idea why he is paying attention to the practice referees, but a lot can be learned by watching a team practice.
We need to be careful while watching training camp practices and assuming we immediately know how good a player is, or will be. Every year there are a couple of players that look amazing in training camp and pre-season games; during light practices and vanilla game plans against lower tier players, that never latch onto a team, or are ultimately released not long into the regular season. The quantifiable things learned during this time are all but completely irrelevant. The fight in a short individual drill, the recovery from a bad play in intrasquad scrimmages (as well as the games themselves), the attempts to improve and learn from the older players, coachability of a player, etc. these things that coaches see day in and day out throughout the season, are the important takeaways from this time of year.
You'll watch some boring stuff, slow walk-throughs of plays that are used for repetition and to make certain movements become second nature, but if you watch the plays then you start to learn them, too. This is what they will be running during the season. So when you're watching a game, and you see Kevin Zeitler and Russell Bodine pull, A.J. Green drag across the formation, and Giovani Bernard flare out, you'll remember watching them do it slowly in practice, and know they're running a halfback screen. Besides, nobody is trying to get injured in training camp.
Two of my favorite things to watch during practices are combined drills for both sides of the line and for cornerbacks against receivers. There's a lot of fire in these competitions, and they're so much more direct.
Here you can see Dominique Rodgers-Cromartie against Mohammad Sanu. It's tight one-on-one coverage, it's a battle between two men for the ball, almost by any means necessary. Sanu made this catch, with Rodgers-Cromartie blanketing him, hand in his face, and ultimately on the ball. If Sanu can do this again all year, it'll be a boon for the Bengals' receiving corps.
During most practices, teams will "scrimmage" each other multiple times. They're not scrimmages that count for points, the yards gained don't matter, there isn't even much hitting outside of the battles in the trenches. These scrimmages are just to line up a play against another player, for the quarterback to read the coverage, the defense to read the play being run by the offense, running backs to read their lineman. It's just another thing that isn't quantifiable, and seeing a running back break down the field, doesn't necessarily mean that running back should be the starter, but in the spur of the moment analysis world we live in, most people automatically anoint them. Defenses are typically not allowed to tackle during these scrimmages, but you can see how the ball is moved, and how the defenders react, and that's exciting.
Training camp involves a lot of playing around with formations and play calls because... Well... They can. Putting players outside of their comfort zone, testing them in different positions, all things that will never show up on a stat sheet. It's part of the strategic war that is football.
Next time you watch a preseason game, don't focus your attention on the yards gained, or the numbers at the end of a drive/quarter. Watch how those yards are gained, the blocking of the lineman, and how the running back finds the hole. Look for where the open guy was when the quarterback threw the ball to him, and how the defenders may have beaten the offensive line to get in the quarterback's face. See how the safeties and linebackers rotate to the ball, changing things before the snap to throw off the reads of offensive players. You will be amazed at how much more you learn than you did by looking at the numbers.