The New York Times' David Kirkpatrick wrote the best, most accessible introduction to Khattala — you can read it here, including some quotes from a sitdown with Khattala himself. Allegedly, Khattala coordinated the attack on the US mission in Benghazi as it happened. Kirkpatrick's sources told him that the militiamen attacking the mission reported to Khattala, calling him "sheikh." Khattala, the sources, say, ordered the militias to "flatten" the mission. He also sent soldiers to attack a nearby CIA compound. (Here's a timeline and map of the attack.)
On Kirkpatrick's telling, Khattala was also a loner, shunned even by other Islamists. "He thinks he owns God and everyone else is an infidel," Fawzi Bukatef, another rebel leader, told Kirkpatrick. He became nationally infamous when General Abdul Fatah Younes, the leading rebel commander, was killed while in Khattala's custody.
This isn't an indefinite Guantanamo detention situation. Now that Khattala is in US custody, the US Attorney's office has filed charges formal charges against him, so if he doesn't confess he'll go on trial for the Benghazi attack.
It's hard to say how much this will matter for disrupting violence inside of Libya. Pinning down Khattala's operational role inside of any of the extremist groups in the country are tricky. It's also not yet clear whether he had plans to conduct further attacks on American targets.
That said, capturing and trying alleged terrorists is a major pillar of American counterterrorism policy. There's a deterrence issue here: if groups can target US facilities abroad without any consequences, it's possible that could make them more likely to risk that kind of attack. Arresting Khattala, if he really was responsible for one of the most high-profile attacks on an American target in recent memory, could aid in the goal of deterring would-be attackers.
The legal process against Khattala could also have enormous political ramifications. On a very basic level, one of the core conservative criticisms of President Obama's handling of the attack is that he hasn't brought anyone to justice over them. Obviously, that argument will have to be modified or discarded.
More subtly, Khattala could provide information about al-Qaeda's role's in the attack. In his interview with Kirkpatrick, Khattala denied any connection to al-Qaeda, though he did express admiration for their methods. Conservatives have long alleged that the administration misleadingly downplayed al-Qaeda's role in the attack; Khattala's interrogation and trial may bring new evidence to light on this front.
Perhaps the most important element in the al-Qaeda controversy is the role of Libyan militant group Ansar al-Sharia. Everyone agrees that Ansar members participated in the attack; the big question is to what extent Ansar can fairly be described as an al-Qaeda branch. According to the State Department, Khattala is now a "senior leader" in Ansar. His capture, then, may shed more light on the group's place in the al-Qaeda network.
Finally, there's the famous controversy over the extent to which the attack was motivated by an anti-Islam film, The Innocence of Muslims. If Khattala really was behind the attack he could certainly illuminate whether the film played into his motivations.