It's not that important what you talk about and what are you doing somewhere for who those systems are used. Just knowing who talk with, and where you're is extremely helpful to law-enforcement to triage from millions of people in the country, to something closer on the order of thousands of people of interest.

Goal of metadata investigation isn't to directly target you, most of the time. It's to put you in the bucket of interesting people, that government will pay attention to.

It's exactly the same as ads on the internet - they maybe classifying you as a person potentially interested in computer security because you're visiting tech crunch. Are all people visiting it interested in computer security? Of course not. But you're many orders of magnitude more likely to be interested in it, than a random person from the internet.