coaches don't need to see how players do these drills in pads. They have poo tons of video, senior bowl, and pro days to watch how they move in gear. The Combine is about...
What is the NFL Scouting Combine?
Back in the 1960s, a few teams started doing more formalized scouting of college players. They quickly figured out that taking empirical measurements of these players' abilities was a helpful exercise, and so they started asking colleges to run quick tests, the 40 yard dash among them. However, teams soon found huge discrepancies in the data that college teams were reporting, and eventually they figured out it was preferential to do these tests on their own. Eventually, this turned into what is now the Combine.
Today, the Combine is a huge deal for both NFL teams and prospects, as it allows teams to run these prospects through assorted tests and drills, verify their height/weight, and probably most importantly, conduct interviews with them. This year, it will be held from February 23 to March 1, as always in Indianapolis.
What are the events at the combine?
Electronically timed, a player runs 40 yards as fast as he can, getting split times at 10 and 20 yards. 4.5 seconds is an elite time, anything less than that is just about superhuman. Chris Johnson of the Titans has the record with the current timing system at 4.24 seconds; Deion Sanders had a 4.21 with a different system, and it was rumored the the amazing Bo Jackson recorded a time of 4.12 seconds, which would have made him probably the fastest person in the world at that time. On the flip side, we have Tom Brady.
In the 40, you are looking not only at absolute speed, but also how the athlete runs: do they accelerate quickly (most important for linemen)? Do they run low and powerfully, or do they get high and stride long? What are their running mechanics like?
Complicated: from a stationary, flat footed start, a prospect jumps as high as they can. Anything above 40 inches is elite. The current record is held by Gerald Sensabaugh of all people, at 46 inches. This is most useful for WRs and TEs, and to a lesser extent DBs.
Interestingly, the second best vertical ever recorded at the combine was a guy who went undrafted: Derek Wake. You know him better as Cameron Wake, Dolphins Pro Bowler.
Standing stationary, a player jumps as far forward as he can. Anything around 11 feet is elite, I think the current record is held by Calvin Johnson at 11'7" (not positive though).
Scouts are not just measuring the distance, they're evaluating how strong the lower body looks, and how effectively the prospect can explode up and out. Sometimes the best performances are not necessarily the furthest, especially if the prospect gets a lot of upward motion and shows a lot of power in doing so.
225 lbs Bench Press
Prospect does as many bench press repetitions as he can with 225 lbs (bar + 4x45 lbs plates, for reference). Anything above 35 or so is elite, the current record is held by a guy name Justin Ernest, who weighed a whopping 281 and did 51 reps.
In general this drill is a pretty good indicator of both functional upper body strength (as the motions are similar to what both offensive and defensive linemen actually do on the field) and of how hard the prospect has worked in the weight room. Elite players, particularly offensive linemen, usually have VERY long arms, so it is more difficult for them to reach into the 40s; scouts account for this, and usually consider, for example, a performance of 35 reps by a guy with an 84 inch wingspan to be more impressive than 42 reps by a guy with a 75 inch wingspan.
20 yard shuttle
3 cones are set up , outer two are 10 yards apart, and one cone is in the middle. Prospect starts in the middle, shuffles sideways to the cone on one side, then shuffles to the cone on the far side, then back to the cone in the middle. The drill tests how quickly a prospect can change directions; a test of agility.
This is particuarly important for offensive linemen and most important for OTs, as it shows how well they can move side to side, a key skill in pass protecting. It also shows how players like DBs and LBs can move laterally. Scouts are looking not only for quick times, but how low a player gets while doing the drill, and how athletic the foot movements are. Anything below 4 seconds amazing, the most impressive I've ever seen was Dante Hall (although his wasn't the fastest ever). Unfortunately no videos exist of it, but it didn't look like a human being doing the drill, it was awesome.
3 cone drill
3 cones are set up in a triangle, the athlete runs around them and backwards and stuff (I won't describe it in detail). Scouts are evaluating agility, ability to change direction, and general footwork.
In general, these results should correlate well to the shuttle. The defensive backs get special attention in this drill, as the very quick changes of direction can show how fluid their hip movements are, a key component of cover technique. Good times are in the 6.7-8 range, elite times are below 6.5 seconds.
Each position has a handful of practice drills that highlight specific position skills: linemen do the "punch and shuffle drill", quarterbacks throw on various routes, linebackers "follow the ball", receivers run routes and catch balls, etc.
These are much less quantitative than the above drills, but they are arguably more important, as it is actual football skills being tested. When you watch these drills, you can really compare players to one another effectively, and it is really goddamn entertaining at the same time.
Complete physical examination, to include being put up on a pedestal and examined like livestock. Looking for injury history evaluation, body composition and physique, and confirming height/weight.
Interviews: teams get 15 minutes to talk to players. Very important event.
Drug screen: idiot screening process. If you can't not smoke pot for a few weeks before the combine, you're going to have a tough time as an NFL player.
Cybex test: creepy injury/joint/flexibility test
Wonderlic Test: very controversial aptitude test; doesn't correlate particularly well to success in the NFL, but teams do pay a bit of attention to it, especially for QBs.
But none of these drills matter! Look at the game film you idiot
Well, yes and no. "Workout warriors" emerge every year, and they have a pretty high bust rate as doing well in these workouts does not correlate perfectly to being a good football player.
However, the combine is still an extremely useful tool. There are a zillion different factors that go into how a player looks in-game, but at the combine, there is just one: that player's ability. Sometimes it confirms the amazing things seen on film (Calvin Johnson, Adrian Peterson), sometimes it reveals a lot about a player's work ethic or lack thereof (Terrence Cody, Alan Branch), sometimes it reveals amazing hidden gems (Chris Johnson, Vernon Davis, Dwight Freeney), sometimes it totally misleads us (Matt Jones, Vernon Gholston), and almost always made Al Davis do dumb things.
In general, football fans today seem to be in something of an anti-combine phase right now: people have decided THE 40 DOESN'T MATTER THEY DON'T EVEN WEAR PADS, and so they're very quick to write off good combine performances. It DOES matter though, you just have to take everything in its proper context.
if you want to watch people competitively run, lift weights, and jump, then find your nearest crossfit gym.