Strictly speaking, it's not a requirement. But it gives incredible cover. It makes everyone think, "well, it's possible - they know everything that goes on."
In other words, absent this sort of mass vacuuming, the individual targeting of a dissident might seem much more suspicious.
Later
BTW: I felt like I responded to what you asked in other comments on the thread, and by the time I saw your comment, many other people had already responded. Nothing personal!
2. some of those very real issues have very real solutions, but people still cry bloody murder because those solutions don't solve every imagined problem.
3. Exaggerating the scope of the problem conveys a sense of helplessness. What purpose is served by making people needlessly afraid? There's a point at which the harm caused by surveillance panic exceeds the harm caused by surveillance.
4. the hysteria results in people making stupid choices regarding email, messaging, etc. like "I can't trust the companies I know, so I'm going to trust companies I don't know". That's ridiculous. Or the fact that while Comcast may inject some janky notifications on web pages I visit, it's not straightup malware like rogue tor exit nodes inject.
5. It becomes impossible for anybody to actually discuss what's happening when every little molehill is a backdoor, or rootkit, or malware, or existential threat to cyber existence.