Why did he have agents at the neighbors two days before announcing the indictment? Thoughts? If he knew Libby had twice lied with documents proving the contrary, why would he still send them?
To make sure the third sentence in paragraph 1.f. of the indictment was not easily shown to be untrue. Reporters would be nosing around Wilson's place after the indictment was handed down, and it would have been quite embarrassing if a reporter asked a neighbor, etc, ... you get the idea, it would be a fact automatically damaging to the prosecutor's credibility.
Fitzgerald gratuitously set-up the meme that Plame was covert, even to the point of putting the phrase "and her employment status was classified" in the indictment. All part of setting a misleading stage. But he had his case against Libby even without that misleading setting.
he wanted to trump up his prose on the outing charge (for which there was no indictment), by trying to show that her identity was not "common knowledge". so he sends the FBI to interview the coached neighbors, so he can say "see, wouldn't the neighbors have known if it were common knowledge?". its pure BS, and it gives great insight into Fitzgerald that he would do that.