Re: And just how does this
At least in theory, they want to snoop on the planning stages. You may say "they can just plan offline", but, well, there's a reason the drones keep hitting weddings 'by mistake': HumInt is tracking when suspects meet, and calling in the hellfires if they think there's enough Target in the collateral.
That sort of thing (which, IMO, is probably some manner of war-crime) strongly discourages personal meetings. So SigInt has to try and fill in the gaps. Your average bomb-maker isn't going to roll their own crypto, but they're happy to use one of the off-the-shelf ones that the Five-Eyes types complain about not being able to crack.
To be clear: I have some sympathy for the aims! Terrorists are bad. Child abuse rings, also bad.
But the NSA, GCHQ and pals took a calculated risk by violating the privacy of millions, and the dice didn't work out for them. Nobody held a gun to their heads and said "you must spy on your own citizens, en mass, on dubious legal grounds". This is their screw-up, and it'd be nice if they took the consequences like adults.