So, after all this, nobody can answer my question as to how participating in track and field events makes you a better football player. Yes, I know a guy who runs fast on a track will probably run fast on a football field but is he fast because he runs track or does he run track because he's fast?
It's a little bit of both. Whether you are fast or slow, there are techniques taught that can increase an athlete's speed. Of course, there is a limit to what proper form and technique can do for someone's speed, and that's where body composition and genetics comes into play.
That being said, for your skill position guys, track can help marginally improve a football player's speed. But track speed does not always translate to game speed. Some players post great numbers in the 40 and 100, but don't play to their speed for certain reasons (usually mental).
Some of the field events (shotput, for instance), are of marginal relevance for football players (DL and OL, usually), in that they require strength, balance, and coordination to perform at a high level.
But at the end of the day, nobody is wearing pads at a track meet or physically lining up across from the track athlete and challenging him, so the significance of track skills on football skills is by no means clear. A kid who does well in track and field is definitely an athlete, but not all athletes are good football players.
IMHO, the best thing track has going for it for a HS football player is it's a spring sport that will keep the athlete working out and regimented in the off season. Of course, the rise of 7 on 7 has minimized this benefit, since skill position kids can work on more football-related activities year round.