1) It's at least partly election year posturing/positioning. I'm willing to bet that neither Hillary nor Obama signed on, but McCain sure did.
2) Congress is trying to get SCOTUS to do the dirty work for them so they don't have to get their hands dirty on a controversial issue. The signatories form a majority of both houses, which means, since the Constitution grants Congress authority over the Federal district, they could kill the law themselves and render the whole thing moot if they wanted to. (Although, conceivably, since they only have 55 Senators, a filibuster would be possible.)
3) I think the brief needs to be read for to see what it actually says, especially if some of the signatories are saying things like this:
Hutchison and Sen. Jon Tester (D-Mont.), who spoke at a Heritage Foundation event yesterday, said the court could find D.C.'s law unconstitutional without another trip through the courts and without endangering Congress's ability to pass other gun control legislation, such as banning assault weapons.
So they're saying that it's the DC Council, whose powers were delegated to it by Congress, can't ban pistols, but Congress can ban assault weapons. Can someone explain that logic to me?
UPDATE: THR member LAR-15 has posted the full brief. Note that in addition to 55 Senators, the President of Senate is also signed on. The President of the Senate is another of saying the Vice President of the United States of America. Thank you, Richard Cheney, for sticking your finger in the eye of the Leftists one more time.