A plug here that the promoted and stated goal "stopping the next 9/11" is a politics of fear and not what ultimately justifies the capabilities to each administration.

Reprised from an earlier comment:

If you look at the Snowden documents (and leaks by others) you'll see essentially nothing other than the international nature of the programs. For example, you'll remember from the Snowden leaks that the NSA hacked the Brazilian oil company PETROBRAS to help American oil companies win offshore oil drilling locations. The hacking of Merkle's cell phone was a big deal because it revealed that the US had information from Germany during the Eurozone crisis! Stuxnet was used to destroy Iran's nuclear program.
The US also faces the same sort of pressure from other countries. This year alone the DoD was hacked, Wall Street, NASDAQ and JP Morgan were hacked and hundreds of defense contractors were hacked - all with foreign attribution. Israel's Iron Dome designs were hacked by China.
Take a look at the NSA program HACIENTA, which "is used to port scan entire countries" and which uses other compromised (civilian) computers to disguise attribution.

Look at The Intercept reporting (where Glenn Greenwald is right now). He speaks at length about how the US uses NSA operations to benefit the global bargaining posture and competitiveness of US companies. https://firstlook.org/theintercept/2014/09/05/us-governments...

And take the Inspector General's report from the Boston Bombings - a great example of how and when the NSA domestic programs would be used if they were about terrorism. The NSA is hardly mentioned. The Inspector General investigates the failings of the FBI. (http://info.publicintelligence.net/IC-IG-BostonBombingReport...)

"We focused our review on the entities that were the most likely to have had information about Tamerlan Tsarnaev prior to the bombings – the FBI, the CIA, DHS, and NCTC, which maintains the U.S. government’s database of classified identifying and substantive derogatory information on known or suspected terrorists. We also requested other federal agencies to identify relevant information they may have had prior to the bombings. These agencies included the Department of Defense (including the National Security Agency (NSA)), Department of State, Department of the Treasury, Department of Energy, and the Drug Enforcement Administration."

The report on the failures to anticipate/stop the Boston Bombers barely mention the NSA. This is because the Federal Bureau of Investigation and the National Counterterrorism Center are in charge of counterterrorism, not the National Security Agency.

Or go to the NSA's own mission statement.

(https://www.nsa.gov/about/mission/index.shtml)

"The National Security Agency/Central Security Service (NSA/CSS) leads the U.S. Government in cryptology that encompasses both Signals Intelligence (SIGINT) and Information Assurance (IA) products and services, and enables Computer Network Operations (CNO) in order to gain a decision advantage for the Nation and our allies under all circumstances."

(Nothing to do with terrorism.)

Lots of news recently has called out Executive Order 12333's role in defining the goal and the means of intelligence capabilities. EO 12333 was passed in 1981. The Five Eyes, the key partnership of the NSA, has its origins in the 40's and ECHELON and other leaked programs (eg CARNIVORE/PREDATOR) predate 9/11 by decades.
The Snowden leaks disclose a list with over thirty countries with competing digital intelligence programs.

The NSA is not about terrorism. Never was. Never will be. The NSA and CSS are the intelligence arm of the United States. Austrilia's programs are similarly not about terrorism. Canada, too. New Zealand? Which terrorists have been attacking New Zealand?!

Digital communications play a huge role in global communications and corporate and international power.
That's not to say there no domestic component to the programs. Domestic programs are also useful to track and disrupt radical ideas and organization within the country (MINERVA), and can also be used to incite discontent in other nations (look up the USAID Cuban Twitter program). Countries are able to manipulate the appearance of consensus within citizens of nations and in this way actually affect this consensus. (Look at the GCHQ programs leaks with BIRDSONG/BADGER/GATEWAY/SLIPSTREAM/ETC.) They also are used to monitor, detect and perform forensics on breaches from other countries.

There's so much to say, but I'll leave the comment with this. Digital communications are so insecure that the attackers always win. Always. And digital communications play a huge role (next to satellite and radio communications) in modern espionage and sabotage. If you just play a defensive game, you lose. The US feels it needs these capabilities for these reasons - not because of terrorism.