First, thanks Zero for posting the Off Season Schedule.
Second, there was a time when players simply reported to Training Camp. Some worked hard during the off season, but a lot of players did not. As a result the rate of pulled muscles, sprains, cramps, and injuries was hurting teams before they could start the reg. season. Off season training programs help to reduce team injuries and has produced a better quality of athlete.
Finally, the NFL realized that they needed to keep their product in the public eye just as MLB was doing in their off season. The result was/is higher ticket sales, higher product sales (ie: Team Jerseys). and higher television ratings. Never forget that the NFL is first and foremost a Business.