30 year old skeletons in people’s closets can be great blackmail to gain leverage with.
edit: As I understand it this is a popular way for state actors to "flip" people. Threaten them with blackmail unless they provide confidential information or do some actions.
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.
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.