I don't know why they bother. All they have to do if they need that info is get a court order and access the records of the various telecom companies. Far easier than allocating the resources of your intelligence apparatus to do it.
They'll claim it's because the "threats" move very fast and the courts would take to much time to grant access to the required intelligence.
Really, it's just so they can bypass the legal system to gain any intelligence they want, be it on terrorists or new political adversaries.