Their mission also explicitly includes information assurance:

Mission Statement The National Security Agency/Central Security Service (NSA/CSS) leads the U.S. Government in cryptology that encompasses both signals intelligence (SIGINT) and information assurance (now referred to as cybersecurity) products and services, and enables computer network operations (CNO) in order to gain a decision advantage for the Nation and our allies under all circumstances.

They've got to balance both roles.

IIRC, in earlier times the government didn't use as much COTS stuff, and civilian computer systems weren't so critical, so the roles were easier to separate. The NSA developed whole series of secret encryption algorithms for the exclusive use of the government/military, and civilian algorithms weren't approved to secure classified communications.

https://en.wikipedia.org/wiki/NSA_cryptography

I always wondered why Barr, Comey and basically every AG I paid attention to, consistently want to break encryption for the populace.

I guess it makes sense proponents of those changes would be ok of breaking it for the proles of they thought their secrets are protected.

You don't see how a lack of critical vulnerabilities is software infrastructure is of benefit to citizens?

No, I don't see how this is part of foreign intelligence/surveillance/espionage work. It is good that these vulnerabilities are fixed, of course. But shouldn't that be at least a separate partially independent branch of the NSA? Otherwise you get a large conflict of interest.