According to studies, players averaged 7.61 games per year during seasons where they missed most or all of training camp. In contrast, those same guys averaged 12.48 games per year in seasons where they had at least most of a full training camp under their belt. Indeed, nothing can prepare the body for the mutilation of an NFL football season quite like the mutilation from the training camp that precedes it.
Another significant benefit to training camp comes with the opportunities it provides for players to learn the system and build a rapport with teammates with whom they specifically may need to be on the same page. This is doubly true for players learning a new system or familiarizing themselves with new teammates.
Finally, you are also able to put yourself in a real game situation. Here is an example of a drill done by Cardinals' Markus Golden where you can see the direct correlation from the drill to gameday.