Generally, it's done for money. Sometimes it's done for a noble cause. I'll list a few local examples of the latter.
The few examples I can name were done for the fans at Thrash of the Titans, the benefit concert for Chuck Schuildner (RIP) and Chuck Billy. It wasn't done for money,cause there ain't no money in what these guys do...
Vio-Lence : Some new members, talent level isn't quite as high, but it's great to hear all the old tunes again... Played a couple shows since reuniting for Thrash of the Titans, due to overwhelming fan response. Maybe they'll keep going.
Forbidden : Was awesome to see them preform live once again. Most of them are all committed to other projects, so it was "one for both Chucks". They haven't played a show as Forbidden since.
Exodus : Reformed for TOTT, with Paul Baloff, and they had plans to come out with another album, and then Paul had to go and die.
RIP PAUL! They Bay just ain't the same without you! Steve Souza has rejoined the band, and it just ain't the same. But it's great one of the founders of thrash are still around.
Death Angel : Another band reunited for the benefit show. They haven't played a show since, but rumor has it they're in the studio and a new album and tour supporting it will follow.
Unfortunately, the rumors that Dark Angel (not local) would reunite, turned out to be just that. There were plans started, but nothing became of them, and a member said on their web site it would never happen. Bummer. They were awesome live. Guess who the member is... (reborn christian... gasp)...
Just the view from my little portion of the world...