We're afraid that trying to understand a terrorist opens the door for us having to say that blowing up civilians really isn't that bad, or understanding a criminal means we have to excuse all his crimes because he had a bad childhood. That is silly. Understanding is just about gaining knowledge. Once you've gained that knowledge, you can decide what to do with it. This includes using that knowledge to defeat them, if that's what the situation calls for. It applies to terrorists or criminals or aggressive co-workers or Internet trolls. You can't fight what you don't understand.
As a wise man once said, "In order to trap him, he must become him."
Everyone quotes this wrong.
Sometimes you need to metaphorically undergo surgery to replace your face with your enemy's face, and (also metaphorically) infiltrate a prison and talk to your enemy's brother in order to find out where the metaphorical bomb is. You don't have to learn any lesson about how he's "really not that bad," and quite likely will learn he's even worse than you thought, murdering all your co-workers and displaying a creepy obsession with peaches.
I'm not going to dive into the debate about whether torture is effective, but I will say that interrogators have "broken" al-Qaida operatives with gestures as simple as bringing sugar-free cookies to a diabetic or finding a suspect's childhood nickname and calling him by it. One link in the chain to capturing al-Qaida's top guy in Iraq involved an American interrogator taking the time to find out why one imam was so pissed at Americans and then just saying sorry.
More effective than waterboarding?