Why does the FBI need to decrypt the phone of a spree shooter?

What could they possible learn, after the fact?

What could they possible know that isn't more easily ascertained by other means?

Every thing about every person is known in real time. If the NSA, FBI, whomever could not identify and thwart undesired behavior with existing tools (follow the money, network analysis, profiling, sentiment analysis, follow the bullets, etc, etc)... Well, they'd have to be unforgivably incompetent.

[I believe they do all these obvious things. Which is why we've seen so few terrorist attacks. (If only they cared as much about spree shooters.)]

Further, what enemy of the state is using COTS communication system? You'd have to be an incredibly stupid drug dealer, human trafficker, money launderer indeed to conduct business via iPhones.

So if the panopticon doesn't and can't help defeat the bad guys, what's the point? Does the government really need to eavesdrop on the selfie nudes my teenagers are sending their friends?