Let's take the worst-case disaster scenario: Terrorists are going to get a nuclear weapon and blow up a major city every year. Millions of deaths per year.

Here are two ways to prevent this:

A) We read everyone's private correspondence without prior suspicion and arrest anyone who we think has obtained nuclear materials necessary to make a bomb. Obviously if we hadn't arrested them they would have killed millions of people, so reading everyone's private correspondence without prior suspicion has saved millions of lives per year.

B) We track down and secure all the nuclear material (e.g. what the Russians have lost) using traditional investigative work so that terrorists can't get it. Since it's too hard for them to make themselves, this prevents them from obtaining a nuclear weapon, problem solved.

C) We send undercover operatives into terrorist networks and disrupt them before they ever get off the ground.

You don't need A if B works. You don't need A if C works. You don't need A if any of D through Z works either.