He was losing ground rapidly as this trumped up "Oh no! If we don't do something immediately the whole economy will collapse!" theme penetrated the culture. Treading water wasn't good enough, the issue was dragging him under. He needed to do something to make voters see him as decisive in the face of a crisis.
His solution was to cancel his campaigning with great fanfare and then, as you point out, accomplish nothing other than support a bill that Obama also supported and 80% of the country didnt. The issue was dead with voters after that, the two were the same but Obama seemed less silly.
McCain wanted to gamble on a game changing moment but then he didnt change the game! Had he come out and said the bailout was unconstitutional and he refused to support it he would have had an issue to go directly against Obama on, a clear contrast. But, he's McCain. The squishy safe middle is where he lives.
he had a game changing moment but the same old policy crap. And don’t forget that he tried to cancel the first debate. And then it came out that he had a katie Couric interview before going to DC.