The purpose of government SIGINT (Signals Intelligence) is certainly not to catch terrorists/pedophiles/money-launderers. Those activities are generally tolerated/endorsed by intelligence agencies, as they are not heinous enough to garner their ire, even helping them whenever they coerce someone into committing a terrorist attack. The true purpose of all of those data is to create a metadata map and to assess who is up to what and who can do what, such that the powers of their nations over the world can be maintained as long as possible.