The wiretapping was transcribed and the names of people who were us citizens hidden , unmasking is the request to reveal the redacted names. These requests prove that the people making them had access to the wiretaps and who was in them

Imagine Watergate, but instead you have proof that a bunch of us officials and the president were reading the wiretaps, including taps of people talking to their kids and of course you also have the dates

Which means you know when they started listening and you can check against when they started investigating ... When they made the documents authorizing this etc

Basically he caught them, he caught them all