Apple likes to have long lines of people camped out in front of their stores before a release. Its the highest caliber of free media coverage available. CNN, MSNBC, all the way down to smaller local news stations will all run a story on the evening news, showing their viewers that the new iPhone has arrived. They'll pan down the line, showing the dozens of people all waiting to own the coolest new piece of tech, and of course they'll have to interview the guy standing in the front of the line, and find out how long he has been camping there.
There is not a commercial Apple could ever release that is as successful as the media blitz on launch day. It shows the world that the new iphone has arrived, it shows that it is the most in demand phone on the market, and it tells the viewer that everybody is literally lining up for a chance to get theirs. Best of all, not a single minute of this coverage costs Apple a single cent.
Would there still be a rush to the store if it was announced that it was available after the unveiling on Tuesday? Of course there would be. But without advance warning, the news media would already have reporters assigned to other stories, and the iPhone may not get the coverage Apple desires.