From a fan-attracting perspective, tourneys always do much better attracting fans and selling tickets when Agassi's playing. I think it's been commentated throughout the past few years that his matches always sell out the quickest and/or gather the most spectators, even when you have people like federer and roddick playing. I used to go to the SAP open each year in CA, and his early round matches would almost be sold out each time; this year he withdrew, but the tourney advisors conveniently decided to delay announcing it to the public because they knew they would lose like 1/2 of their sales if he didn't come. So just from virtue of him being a crowd-gatherer, he has given back to tennis- he's popular. people come to see him. He gets people to the tournaments, and keeps them interested and coming back to see tennis.
I think they mentioned that Agassi's first round match at the US open this year had something like a 150% increase in viewership as opposed to last year.... and his next two matches also had a much larger tv audience than normal. So in that way, in his 30s, he attracted money and popularity to the game of tennis, maybe even as much as he did in the early 90s... so maybe not so much how he dominated on court, but what he brought to the court.
And not going into his reasons for continuing to play- either selfishly or for the fans, you can see by wimbledon and the US open that he at least said he wanted to say goodbye to his fans, etc. and didn't have any realistic hope of going far with his back injury. he was doing it because (in his words) he wanted to "give back to the sport that's given so much to me"- he thinks it's his duty to play as long as he can, for the fans and for the sport, and for his charity. I know this wasn't a great year, but I still think it's admirable he didn't want to just "leave on the top" last year- he recognizes what tennis has done for him, and he wanted to leave it all out there.
and in addition to his academic school, he does have a tennis charity in Vegas too i believe- it works with inner city youth. he's spoken a lot about kids getting the opportunity to pick up rackets that wouldn't otherwise over the years, so he's passionate about getting more people interested in tennis/gives back that way.
i think a lot of it probably has to do with the fact that he's just so well-known, charismatic, etc. and that he can be credited with a lot of the interest in US men's tennis over the years (esp. after the rest of his generation retired.) He always connects with the crowd on a level above everyone else- you saw that 8 minute goodbye ovation.
Plus (last thing
), he could've easily retired 8 years ago when everyone thought he was over, but he worked his way back to the top- probably would've been at the top even now if not for his back injury- i think just his amazing comeback, and his ability to stay at the top into his 30's is one of those famous/feel-good stories of the tennis community, and not something others have accomplished.. Wasn't Agassi the oldest ranked #1 at 33 or something?
i'm not trying to worship the guy, it's just what i came up with as possible reasons.