The data this actor wants may be in an air gapped secure facility, for example Iran's nuclear facilities. Decrypting old social media messages that show that a scientist at that facility had a homosexual relationship while he was in college in Europe would give you access in a way you didn't have before.

That is an extreme example but high value information is often stored and secured in ways that are very resistant to theft. Using less secure and/or historical data to gain leverage over those with access to that data is exactly how spies have been doing things for centuries.

a) Yes. Plus most all digital mediated activity (phone calls, texts, location, purchasing history, yadda yadda).

b) Astronomy has (had?) the same conundrum: gathering acres of data, most of which will never be examined. (Don't call attention to yourself and hopefully law enforcement will ignore you.) Alas, now we're creating tools for chewing thru big data(s), to spot patterns and anomalies. For better or worse.