Frankly, the NSA and the CIA are redundant, and only one of them needs to exist.
I think the NSA just gathers information electronically. They don’t have any human intel sources. I don’t think they attempt to draw any major conclusions from their data.
The CIA allegedly does two things: gather intelligence from both spies and electronic intelligence, and then draws conclusions from the intel to advise the people making policy. The NSA only does the electronic intelligence part afaik. I think the CIA relies on the NSA for much of their electronic intelligence.
Of course the CIA has been almost useless at what it is allegedly there for, as Faddis points out. The lunatics are running the asylum so they may not even define their job as stopping attacks like 9/11 at this point.