Political intrigue, and/or job preservtion.
On the political angle, if the story comes out that the "Bush WH" was leaking to manipulate public perception aghainst Wilson, it plays against the WH.
On the job preservation angle, the WH has a reputation for trying to be "tight," and if he was leaking, then perhaps the conduct is outside the boundary of "acceptable."
There could also be a "get the media in trouble, just for fun" angle. Libby's testimony to investigators would, if believed, cast blame on leaking solely to reporters.
Fitgerald's repeated use of the word "classified" is and was a red herring.
The Identies Act requires that Plame have been "covert" (not "classified") in order for there to have been a crime. And for the Act to be violated, Libby or anyone else would have to have *known* she was "covert", and furthermore must have obtained that information from other classified sources.
This is a pretty high bar, but it's really because the Act is intended to protect foreign covert agents from the kind of outing that Phil Agee (a former CIA agent) had been doing in the 70's. Agee used his classified status to "out" literally hundreds of covert agents.
Also there are tens of thousands of people in the DC area with "classified" jobs and positions, but they are in no way "covert" or protected by the Act. It's not prudent to do so, but if a government employee states that they work at NSA or the FBI or the Pentagon and that their work is classified, no crime has been committed. If Joe Schmoe tells a reporter that his neighbor John Smith has a classified job at the State Department, Joe Schmoe has committed no crime whatsoever.