As others have pointed out, there are three sets of tapes: The Jay Glazer tape, taken from Matt Estrella during the game 1 in 2007, the Matt Walsh tapes, and the tapes destroyed by the NFL at Gillette in 2007.
The Glazer and Walsh tapes were shown on TV and show the same type* of thing: A repetition of the scoreboard (for time, down and distance, etc) and the defensive signals being sent in from the sideline. Some of the Walsh tapes were more of a finished product, editing the signals in to coincide with the game footage. It stands to reason that the tapes the Patriots turned over to the NFL were the same as Walsh tapes, defensive signals matched up with game action.
It's important to note that the "finished product" tapes were of the entire game. If Patriots goal was to decipher the signals after the first quarter or half time, it wouldn't make any sense to tape signals the entire game or go to the trouble of editing them after. They would have found what they needed during the game. That they taped the whole game fits in with Belichick's interpretation of the rule. He thought he was okay as long as nothing was being done with the footage during the game. As it turned out, he was wrong, but Matt Walsh's interviews also support this. He was consistent in telling Specter, Goodell, the New York Times, etc, that he filmed the entire game and only turned the film over to someone after it was over. In fact, he said, in some cases, he wouldn't give the tapes to anyone else until a day or two after the game. The idea that the Patriots were gaining an in-game advantage isn't supported by any real evidence. In terms of answering the "If it didn't help, why did they keep doing it?" question, it helped them prepare for the next time they would face that team or coach or coordinator.
Something else that doesn't add up to me about the "NFL found more incriminating stuff and destroyed it to protect the Patriots" theory is why would the Patriots hand over any more "incriminating stuff?" They knew the league was coming for a visit and knew what they were looking for. Hypothetically, if they had stolen playbooks, hidden cameras, and mind control devices, why would they tell the NFL about them? If I was pulled over for speeding, why would I tell the police officer that I was also embezzling money from work? If the Patriots really were this corrupt and devious organization people believe they are, I don't think they would just crack at the first sign of trouble.
*Some shots of cheerleaders were also included.