An assesment of threat.
There are undoubtedly people who for whatever reasons want to get their own way by indiscriminate violent means. The logic being, that if by perpetrating atrocities, they can show the elected authorities to be ineffective, public opinion will force the elected to give in. It rarely works and usually what happens is; after a ceasefire period talks take place with an implicit threat of chaos may be resumed.
Targeted eavesdropping may well uncover some plans but for all their moral emptiness the organisers of atrocities are not dumb. They will come up with alternative methods of encouragement and communication. The latest 'lone wolf' truck assaults show this. I doubt any form of Internet monitoring would have spotted these individuals before the event.
Like most people I don't think I have anything to hide, the odd emabarassing web search maybe, but I am concerned that such a law could be severely misused.
Imagine if the UK was in a state of emergency as France has been and still is. Our Home secretaries are controlling enough in normal times.