Terrorism is part of it, but it goes back long before that and it's certainly not the main or original purpose. Though I assume in current times it's definitely a "focus" and can be used for it. Many of the signals intelligence programs were birthed after WWII and rose to power during the cold war. They were looking for spies, and conversely looking for traffic to see if anyone was onto our spies over there. They were looking to get advanced warning of attacks, or plans. They were looking to get a leg up on their enemies of the time.

I think above all, it's about security and paranoia, but not in the traditional sense of terrorism or physical security. It's about corporate and government espionage. It's about protecting our secrets and helping root out spies, like the ones that still managed to get back to China with laptops and hard drives from Los Alamos. It's about knowing what our enemies are doing both abroad and at home. It's about knowing what our allies are doing, both at home and abroad. It's about knowing what products or killer apps another country are inventing or creating. It's about the strength of the dollar and the monetary policies of our competitors in currency markets. It's about arms trades to nations that are on our shit-list, and who is providing them. And we could go on and on.

I think at this point in time these programs are scrapping as much as they can, with a whole "We don't know what we're looking for, but we can probably find all kinds of great data in these collections on all kinds of things." It's not even just about snooping. It's about national trends, global trends. They know what the public is talking about, they know what the world is talking about, they see trends coming probably years before they hit mainstream.

I'm just rambling, but just want people to realize it's for all kinds of things. It's not just about snooping on Americans emails or texts, but it also certainly isn't mostly about terrorism either. It's more "What can't we do with this?"

Sources:

ECHELON, one of the older programs, has a few known cases of espionage:

http://en.wikipedia.org/wiki/ECHELON#Examples_of_political_e...

http://en.wikipedia.org/wiki/ECHELON#Examples_of_industrial_...

Total Information Awareness: (some of these capabilities are scary)

http://en.wikipedia.org/wiki/Information_Awareness_Office

Also, to quote the West Wing CIA lady here, "Our failures are public, our successes are not." I don't think that the government feels the need or cares to offer up it's successes. They don't want any potential enemies to know where they fucked up or how. They don't even want the world to know they got caught or foiled. Many are probably paid and turned into double agents, which happened A LOT during the search for Bin Laden, and happens probably even more in government and corporate espionage areas.