Have you ever noticed that people say China is bad because they do massive surveillance on their citizens and that would be illegal in the US? And that for example the police or whichever authority is doing the surveillance needs warrants?
As it turns out, a federal court has ruled that several of the NSA's actions (which had been completely secret until Snowden leaked a bunch of documents) have been illegal. For example the NSA program to collect phone records for US citizens have been ruled illegal.
When it first was discovered that the NSA did these things, they argued that it had stopped terrorist attacks and therefore it was a good thing.
However, as it turns out, in the case of the phone record account findings by the federal court is that the program prevented 0 attacks. The NSA only had a single case where they could argue the illegally collected phone records had helped them stop an attack, but new findings shows that even in that particular case the phone records did not play a major role because they already had a ton of other evidence that lead to a conviction.
Also, last year the NSA built a data center in Utah which cost 1.5 billion dollars and is estimated to hold around 12 exabytes of data
(12,000,000,000 GB). The public are not allowed to know what the data center is being used for, but considering NSA's very troubling hoarding behaviors my guess (and pretty much everyone else's) is that they are using it to store (illegally or morally questionable) personal information about people in and outside of the US.