In a crime case, investigators don't have access to "the truth"—the data, if you will. All they have are clues which can be put together to make as perfect a guess as possible as to what the nature of the truth is. Metadata.
In the U.S, governments have played coy and attempted to talk down efforts of mass surveillance, particularly phone surveillance, by asserting that the actual contents of the call are not collected—only the metadata is:
And that's precisely why governments collect this kind of information: it is powerful fodder in a legal setting. In an example most of us are probably familiar with, you can see that who Adnan Syed called and when were some of the most important deciding factors in placing him in a jail cell.
And as it can be used for legal justice in some cases, or the "good", it can also be used against you, like you're warned of in your Miranda rights.
We protect ourselves and the people we care about, not against the government, but the possibility of government. We don't speak unless we have an attorney present. We plead the fifth.